Debt - Outstanding Convertible Note Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liability component:
Principal	$ 816,417
Net carrying amount	734,683	$ 986,521
Convertible Debt
Liability component:
Principal	341,417
Less: debt discount and issuance costs, net	73,025
Net carrying amount	268,392
Gross equity component	82,481
2022 Convertible Notes | Convertible Debt
Liability component:
Principal	320,000
Less: debt discount and issuance costs, net	71,806
Net carrying amount	248,194	0
Gross equity component	72,576
2019 Convertible Notes | Convertible Debt
Liability component:
Principal	21,417
Less: debt discount and issuance costs, net	1,219
Net carrying amount	20,198	$ 179,363
Gross equity component	$ 9,905