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Convertible Senior Notes (Carrying Amounts of Liability and Equity Components - Additional Information) (Details) - Convertible Senior Notes [Member] - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Aug. 31, 2018
Aug. 24, 2018
Jun. 30, 2014
2028 Convertible Senior Notes [Member]          
Debt Instrument [Line Items]          
Debt instrument, interest rate     2.00% 2.00%  
Transaction costs related to the equity component $ 6,163        
Remaining period over which the unamortized debt discount will be amortized 10 years        
2019 Convertible Senior Notes [Member]          
Debt Instrument [Line Items]          
Debt instrument, interest rate         2.25%
Transaction costs related to the equity component $ 236 $ 236      
Remaining period over which the unamortized debt discount will be amortized 9 months 18 days