Debt - Outstanding Convertible Note Balances (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Liability component:
Principal	$ 816,417
Long-term Debt	738,563	$ 734,683
Convertible Debt
Liability component:
Principal	341,417
Less: debt discount and issuance costs, net	69,449
Long-term Debt	271,968
Equity Component	82,481
Convertible Debt | 2022 Convertible Notes
Liability component:
Principal	320,000
Less: debt discount and issuance costs, net	68,492
Long-term Debt	251,508	248,194
Equity Component	72,576
Convertible Debt | 2019 Convertible Notes
Liability component:
Principal	21,417
Less: debt discount and issuance costs, net	957
Long-term Debt	20,460	$ 20,198
Equity Component	$ 9,905