Debt - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Amortization of debt discount	$ 38,298	$ 37,085	$ 36,013
Senior Notes Due 2019
Debt Instrument [Line Items]
Contractual interest expense	7,187	7,187	7,188
Amortization of debt issuance costs	3,959	3,863	3,974
Amortization of debt discount	34,018	33,014	32,039
Interest expense	$ 45,164	$ 44,064	$ 43,201