Quarterly report pursuant to Section 13 or 15(d)

Stock-based Compensation - Additional Information (Details)

v3.19.1
Stock-based Compensation - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended
Dec. 31, 2017
Mar. 31, 2019
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense   $ 1,728,000 $ 1,416,000
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  
Options exercised, aggregate intrinsic value   $ 200,000 $ 1,000
Weighted-average grant-date fair value   $ 11.27 $ 18.69
Unrecognized compensation expense related to stock options   $ 7,100,000  
Weighted average period of unrecognized compensation expense expected to be recognized   2 years 1 month 6 days  
Options granted   349,979  
Performance Shares [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense related to stock options   $ 200,000  
Number of options outstanding, non-vested options   41,250  
Stock-based compensation expense   $ 56,000  
Grant date fair value   $ 200,000  
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  
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  
Non Executive Employee [Member] | Restricted Stock Units (RSUs) [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Vesting period   3 years  
Directors [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Vesting period   1 year  
Directors [Member] | Restricted Stock Units (RSUs) [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Vesting period   1 year  
Executives [Member] | Performance Shares [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Options granted 130,000    
Stock options cancelled     50,000
Executives [Member] | Service-Based Stock Options [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Options granted     80,000
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%