Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Coupon interest	$ 172,635	$ 174,027	$ 173,484
Convertible notes payable
Debt Instrument [Line Items]
Coupon interest	8,697	8,697	8,697
Amortization of discount	13,586	12,625	11,732
Interest expense related to convertible notes	$ 22,283	$ 21,322	$ 20,429