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Convertible Senior Notes - Summary of Notes (Detail) - Convertible Debt [Member]
9 Months Ended
Sep. 30, 2017
USD ($)
Aug. 21, 2017
USD ($)
Aug. 18, 2014
USD ($)
Mar. 17, 2014
USD ($)
March 2019 Notes [Member]        
Debt Instrument [Line Items]        
Principal Amount $ 143,750,000     $ 143,750,000
Coupon Rate 5.50%     5.50%
Effective rate 6.25%      
Conversion rate 57.3034      
Remaining Period of Amortization 1 year 5 months 16 days      
August 2019 Notes [Member]        
Debt Instrument [Line Items]        
Principal Amount $ 111,000,000   $ 111,000,000  
Coupon Rate 5.50%   5.50%  
Effective rate 6.50%      
Conversion rate 57.3034      
Remaining Period of Amortization 1 year 5 months 16 days      
August 2022 Notes [Member]        
Debt Instrument [Line Items]        
Principal Amount $ 230,000,000 $ 230,000,000    
Coupon Rate 4.75% 4.75%    
Effective rate 5.72%      
Conversion rate 50.2260      
Remaining Period of Amortization 4 years 10 months 24 days      
2019 and 2022 Notes [Member]        
Debt Instrument [Line Items]        
Conversion rate basis, principal amount $ 1,000,000