Exchangeable Senior Notes - Carrying Amount of Equity Component and Principal Amount of Liability Component (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Exchangeable senior notes repurchased	$ 604,276	$ 610,314
Operating Partnership
Debt Instrument [Line Items]
Carrying amount of equity components	22,597	22,597
Unamortized debt issuance costs	(6,620)	(9,033)
Net carrying amount of liability components	604,276	610,314
Operating Partnership | 2013 Notes
Debt Instrument [Line Items]
Carrying amount of equity components	0	0
Exchangeable senior notes repurchased	49,259	63,170
Unamortized discount - equity components	(315)	(1,187)
Unamortized cash discount	74	281
Unamortized debt issuance costs	(35)	(43)	$ (605)		$ (1,672)
Operating Partnership | 2015 Notes
Debt Instrument [Line Items]
Carrying amount of equity components	22,597	22,597
Exchangeable senior notes repurchased	575,000	575,000
Unamortized discount - equity components	$ (12,974)	$ (17,355)
Unamortized debt issuance costs				$ (11,992)