Debt - Schedule of Total Interest Expense Recognized Related to the Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Contractual interest expense	$ 10,935	$ 8,961	$ 5,000
Amortization of debt issuance costs	1,403	1,275	1,072
Amortization of debt discount	13,414	11,071	7,544
Total interest expense	$ 25,752	$ 21,307	$ 13,616