Convertible Senior Notes - Schedule of Debt (Details) - Senior Notes - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2016
Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Principal	$ 402,500,000	$ 402,500,000	$ 402,500,000
Less: Debt discount, net	(80,408,000)	(90,552,000)
Net carrying amount	322,092,000	311,948,000
Convertible Senior Notes Due 2022
Debt Instrument [Line Items]
Principal	399,997,000	399,997,000
Less: Debt discount, net	(52,365,000)	(62,747,000)
Net carrying amount	$ 347,632,000	$ 337,250,000