Convertible Senior Notes - Schedule of Convertible Senior Notes (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Feb. 28, 2014
Liability component:
Principal	$ 690,000,000	$ 690,000,000	$ 690,000,000
Less: debt discount and issuance costs, net of amortization	(21,255,000)	(27,087,000)
Net carrying amount	668,745,000	0
Net carrying amount	0	662,913,000
Convertible senior notes
Liability component:
Equity component:	$ 101,276,000	$ 101,276,000