Debt - Schedule of Convertible Senior Notes (Details) - Convertible Debt - USD ($)	Sep. 30, 2018	May 31, 2018	Dec. 31, 2017	Feb. 28, 2014
2025 Notes
Liability component:
Principal	$ 1,150,000,000	$ 1,150,000,000
Less: debt discount and issuance costs, net of amortization	(285,321,000)
Net carrying amount	864,679,000
Equity component:	285,225,000
2019 Notes
Liability component:
Principal	690,000,000		$ 690,000,000	$ 690,000,000
Less: debt discount and issuance costs, net of amortization	(9,436,000)		(27,087,000)
Net carrying amount	680,564,000		662,913,000
Equity component:	$ 101,276,000		$ 101,276,000