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Convertible Senior Notes (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Jun. 30, 2014
Jul. 31, 2013
Debt Instrument [Line Items]        
Convertible senior notes, principal amount $ 155,728 $ 206,865    
Convertible senior notes, debt issuance costs 2,467 3,858    
Convertible senior notes, net of debt issuance costs 153,261 203,007    
4.25% Convertible Senior Notes (due 2018)        
Debt Instrument [Line Items]        
Convertible senior notes, principal amount 42,728 93,865   $ 100,000
Convertible senior notes, debt issuance costs 298 1,098    
Convertible senior notes, net of debt issuance costs 42,430 92,767    
4.875% Convertible Senior Notes (due 2020)        
Debt Instrument [Line Items]        
Convertible senior notes, principal amount 113,000 113,000 $ 115,000  
Convertible senior notes, debt issuance costs 2,169 2,760    
Convertible senior notes, net of debt issuance costs $ 110,831 $ 110,240