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Convertible Senior Notes - Additional Information (Detail)
3 Months Ended
May 24, 2016
USD ($)
d
$ / shares
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]        
Loss on conversion of senior convertible notes   $ (1,000)    
Accretion of the debt discount   $ 1,036,000 $ 970,000  
2.125% Convertible Senior Notes due 2021        
Debt Instrument [Line Items]        
Notes issued $ 115,000,000     $ 11,000
Notes, interest rate 2.125% 2.125%    
Proceeds from issuance of convertible senior notes, net of costs $ 111,100,000      
Notes, frequency of periodic payment   Semi-annually    
Notes, date of first required payment   Dec. 01, 2016    
Notes, due date   Jun. 01, 2021    
Loss on conversion of senior convertible notes   $ (1,000)    
Notes conversion ratio per $1,000 principal amount 31.1813      
Notes initial conversion price | $ / shares $ 32.07      
Debt covenants debt default holder percent to declare all notes due minimum 25.00%      
Number of days within which entity fails to satisfy obligations considered as event of default 270 days      
Notes issued, fair value $ 96,289,000      
Contractual coupon interest   611,000    
Accretion of the debt discount   881,000    
Amortization of the debt issuance costs   $ 155,000    
Effective interest rate on the Notes   6.60%    
Notes, carrying value   $ 100,276,000   $ 99,250,000
Fair value of the note   $ 150,500,000    
2.125% Convertible Senior Notes due 2021 | On any business day on or after June 5, 2019 and prior to the maturity date        
Debt Instrument [Line Items]        
Notes threshold percentage of stock price trigger 130.00%      
Notes threshold trading days | d 20      
Notes threshold consecutive trading days | d 30      
Notes redemption price 100.00%