Annual report pursuant to Section 13 and 15(d)

Compensation and Other Benefit Plans - Additional Information (Details)

v3.10.0.1
Compensation and Other Benefit Plans - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2017
May 31, 2017
Feb. 28, 2017
May 31, 2016
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Apr. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Maximum annual contribution per employee (in hundredths)           50.00%    
Maximum annual contribution per employee           $ 18,500 $ 18,000  
Maximum annual contribution per employee over 50 years old           $ 24,500 $ 24,000  
Age requirement for participant to be eligible for a catch-up contribution, minimum           50 years 50 years  
Employer matching contribution percentage, maximum (in hundredths)           50.00%    
Deferred savings plan expense           $ 100,000 $ 0  
Deferred savings plan expense paid in common shares (in hundredths)           100.00%    
Number of shares of common stock to be issued upon exercise of options and RSUs outstanding (in shares)           1,627,591    
Stock-based compensation expense           $ 3,902,000 7,301,000  
Non Executive Employee [Member] | Minimum [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Vesting period           3 years    
Non Executive Employee [Member] | Maximum [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Vesting period           4 years    
Directors [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Vesting period           1 year    
Stock Options [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Minimum age required for employees to qualify for immediate vesting of RSUs           55 years    
Threshold years required for retirement age           70 years    
Number of stock options granted to members of board of directors, executives, and non-executive employees           305,708    
Options exercised, aggregate intrinsic value           $ 1,100,000 $ 2,400,000  
Weighted-average grant-date fair value           $ 18.25 $ 10.26  
Unrecognized compensation expense related to stock options           $ 5,000,000    
Weighted average period of unrecognized compensation expense expected to be recognized           1 year 9 months 18 days    
Performance Shares [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Number of options outstanding, non-vested options           41,250    
Number of options vested           41,250    
Stock-based compensation expense           $ 200,000    
Grant date fair value           $ 200,000    
Performance Shares [Member] | Executives [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Number of stock options granted to members of board of directors, executives, and non-executive employees 130,000              
Stock options cancelled         50,000      
Service-Based Stock Options [Member] | Executives [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Number of stock options granted to members of board of directors, executives, and non-executive employees         80,000      
Restricted Stock Units (RSUs) [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Minimum age required for employees to qualify for immediate vesting of RSUs           55 years    
Threshold years required for retirement age           70 years    
Weighted average period of unrecognized compensation expense expected to be recognized           9 months 18 days    
Total grant-date fair value of RSUs, vested           $ 300,000 $ 2,300,000  
Unrecognized compensation expense related to employee RSUs           $ 2,400    
Restricted Stock Units (RSUs) [Member] | Non Executive Employee [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Vesting period           3 years    
Restricted Stock Units (RSUs) [Member] | Directors [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Vesting period           1 year    
2015 ESPP [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Percentage related to employees to purchase shares at the lower fair market value at offering period           85.00%    
Common stock reserved for future issuance (in shares)           15,000    
Percentage of compensation of eligible employees to purchase shares of entity common stock at discount through payroll deductions           10.00%    
Shares purchased (in shares)           2,948 5,314  
2015 ESPP [Member] | Common Stock [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in aggregate number of shares authorized for issuance   250,000            
Shares authorized for issuance   265,000           15,000
Increase in shares per person award limits   2,500            
2010 Plan [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in aggregate number of shares authorized for issuance       170,000        
Shares authorized for issuance       1,108,560        
Expiration period           ten years from the date of the grant or three to six months from the date of termination of employment (longer in case of death or certain retirements).    
Vesting period           3 years    
Number of stock options granted to members of board of directors, executives, and non-executive employees   998,000 1,018,000          
2010 Plan [Member] | Options and Stock Appreciation Rights [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in shares per person award limits   2,000,000            
2010 Plan [Member] | Other Types of Stock Awards [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in shares per person award limits   2,000,000            
2010 Plan [Member] | Common Stock [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in aggregate number of shares authorized for issuance   1,470,502            
Shares authorized for issuance   2,579,062            
2010 Plan [Member] | Common Stock [Member] | Incentive Stock Options [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in aggregate number of shares authorized for issuance   2,004,087            
Shares authorized for issuance   2,579,062            
Long Term Incentive Plan [Member]                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Shares available for grant           341,540