Exchangeable Senior Notes - Carrying Amount of Equity Component and Principal Amount of Liability Component (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Apr. 30, 2016	Feb. 29, 2016	Dec. 31, 2015	Sep. 30, 2015	Sep. 15, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Exchangeable senior notes repurchased	$ 610,314			$ 623,863
Operating Partnership [Member]
Debt Instrument [Line Items]
Carrying amount of equity components	22,597			22,597
Unamortized debt issuance costs	(9,033)			(11,698)
Net carrying amount of liability components	610,314			623,863
Operating Partnership [Member] | 2.375% Exchangeable Senior Notes due 2033 (the 2013 Notes) [Member]
Debt Instrument [Line Items]
Carrying amount of equity components	0			0
Exchangeable senior notes repurchased	63,170	$ 2,555	$ 19,639	85,364		$ 164,636
Unamortized discount - equity components	(1,187)			(2,605)
Unamortized cash discount	281	72	716	633		6,931	$ 3,750
Unamortized debt issuance costs		$ (7)	$ (36)			$ (605)	$ (1,672)
Operating Partnership [Member] | Exchangeable Senior Notes 3.125% due 2035 (the 2015 Notes) [Member]
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	$ (17,355)			$ (21,565)
Unamortized debt issuance costs					$ (11,992)