Debt - Schedule of Total Interest Expense Recognized Related to the Convertible Notes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Amortization of debt discount	$ 0	$ 0	$ (12,491)
Convertible Debt
Debt Instrument [Line Items]
Contractual interest expense	8,961	5,000	5,000
Amortization of debt issuance costs	1,275	1,072	985
Amortization of debt discount	11,071	7,544	6,927
Total interest expense	$ 21,307	$ 13,616	$ 12,912