Quarterly report pursuant to Section 13 or 15(d)

Restructuring Charges - Additional Information (Details)

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Restructuring Charges - Additional Information (Details)
 3 Months Ended 9 Months Ended
Dec. 19, 2016
Employee
Sep. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Employee
Sep. 30, 2016
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
 [1]
Restructuring Cost And Reserve [Line Items]              
Restructuring (charges) credit   $ 29,000 $ (3,451,000) $ (15,000)      
Accrued restructuring expenses   444,000 $ 444,000       $ 3,594,000
2016 Restructuring [Member]              
Restructuring Cost And Reserve [Line Items]              
Number of employees terminated | Employee 57            
2017 Restructuring [Member]              
Restructuring Cost And Reserve [Line Items]              
Number of employees terminated | Employee     5        
2017 and 2016 Restructuring [Member]              
Restructuring Cost And Reserve [Line Items]              
Payments for restructuring expenses     $ 6,600,000        
Accrued restructuring expenses   400,000 400,000        
2017 and 2016 Restructuring [Member] | Severance, Other Termination Benefits and Outplacement Services [Member]              
Restructuring Cost And Reserve [Line Items]              
Restructuring (charges) credit   $ 29,000 $ (3,500,000)        
2017 and 2016 Restructuring [Member] | Forecast [Member]              
Restructuring Cost And Reserve [Line Items]              
Expected remaining future payments for restructuring           $ 300,000  
2017 and 2016 Restructuring [Member] | Forecast [Member] | Executive Severance [Member]              
Restructuring Cost And Reserve [Line Items]              
Expected remaining future payments for restructuring         $ 100,000    
[1] The condensed consolidated balance sheet as of December 31, 2016 has been derived from the audited consolidated financial statements as of that date included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016.