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Convertible Senior Notes - Narrative (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Aug. 18, 2014
Mar. 17, 2014
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Debt Instrument [Line Items]              
Convertible senior notes, carrying value     $ 250,949,000   $ 250,949,000   $ 249,994,000
Company share price (in dollars per share)     $ 18.55   $ 18.55    
Convertible Debt [Member] | 2019 Notes [Member]              
Debt Instrument [Line Items]              
Equity component of the notes         $ 11,445,000    
Interest expense on debt     $ 3,503,000 $ 3,503,000 7,006,000 $ 7,006,000  
Additional non-cash interest expense     913,000 $ 883,000 1,814,000 $ 1,759,000  
Convertible Debt [Member] | March 2019 Notes [Member]              
Debt Instrument [Line Items]              
Principal amount   $ 143,750,000 $ 143,750,000   $ 143,750,000    
Coupon rate   5.50% 5.50%   5.50%    
Proceeds from issuance of convertible senior notes   $ 139,037,000          
Convertible senior notes, carrying value     $ 142,007,000   $ 142,007,000    
Unamortized discount     1,743,000   1,743,000    
Convertible Debt [Member] | August 2019 Notes [Member]              
Debt Instrument [Line Items]              
Principal amount $ 111,000,000   $ 111,000,000   $ 111,000,000    
Coupon rate 5.50%   5.50%   5.50%    
Proceeds from issuance of convertible senior notes $ 109,615,000            
Convertible senior notes, carrying value     $ 108,942,000   $ 108,942,000    
Unamortized discount     $ 2,058,000   2,058,000    
Conversion rate basis, principal amount         $ 1,000,000