Exchangeable Senior Notes - Repurchase of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 21, 2013
Debt Instrument [Line Items]
Total consideration paid for repurchase	$ 1,109,854	$ 1,088,679	$ 1,122,442
2013 Notes
Debt Instrument [Line Items]
Principal amount repurchased	49,259	13,911	22,194
Extinguishment of liability component	49,019	13,692	21,363
Reacquisition of equity component	31,251	6,350	13,898
Total consideration paid for repurchase	80,270	20,042	35,261
Extinguishment of liability component	(49,019)	(13,692)	(21,363)
Discount on exchangeable senior notes	(230)	(184)	(788)	$ (3,750)
Related debt issuance costs	(10)	(35)	(43)	$ (1,672)
Gain/(loss) on repurchase	$ 0	$ 0	$ 0