Debt - Schedule of Interest Expense (Details) - Convertible Notes and 2027 Notes - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Contractual interest expense	$ 40,151	$ 11,406	$ 7,187
Amortization of debt issuance costs	4,663	4,050	3,863
Amortization of debt discount	34,228	34,039	33,014
Interest expense	$ 79,042	$ 49,495	$ 44,064