Convertible Senior Notes - Schedule of Fair Value (Details) - Level 2 - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
2022 Notes
Debt Instrument [Line Items]
Fair value	$ 897,778
2018 Notes
Debt Instrument [Line Items]
Fair value	$ 1,015,554	$ 681,375

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

No definition available.

Name:	us-gaap_ConvertibleDebtFairValueDisclosures
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant