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Convertible Senior Notes (Carrying Amounts of Liability and Equity Components - Additional Information) (Details) - Convertible Senior Notes [Member] - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
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% 2.00% 2.00%  
Transaction costs related to the equity component $ 6,163        
Remaining period over which the unamortized debt discount will be amortized 9 years 2 months 12 days        
2019 Convertible Senior Notes [Member]          
Debt Instrument [Line Items]          
Debt instrument, interest rate 2.25%       2.25%
Transaction costs related to the equity component $ 236 $ 236