Convertible Senior Notes - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Amortization of debt discount			$ 20,526	$ 20,325
Senior Notes | Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Contractual interest expense	$ 2,768	$ 2,767	8,302	8,293
Amortization of debt discount	3,461	3,205	10,143	9,384
Total	$ 6,229	$ 5,972	$ 18,445	$ 17,677
Effective interest rate of the liability component	7.50%	7.50%	7.50%	7.50%
Senior Notes | Convertible Senior Notes Due 2022
Debt Instrument [Line Items]
Contractual interest expense	$ 2,500	$ 2,500	$ 7,500	$ 7,500
Amortization of debt discount	3,535	3,298	10,383	9,690
Total	$ 6,035	$ 5,798	$ 17,883	$ 17,190
Effective interest rate of the liability component	6.50%	6.50%	6.50%	6.50%
Senior Notes | Convertible Senior Notes Due 2017
Debt Instrument [Line Items]
Contractual interest expense	$ 0	$ 0	$ 0	$ 315
Amortization of debt discount	0	0	0	1,251
Total	$ 0	$ 0	$ 0	$ 1,566
Effective interest rate of the liability component	0.00%	0.00%	0.00%	6.02%