Exchangeable Senior Notes - Schedule of Information about Total Carrying Amounts of Equity Components, Principal Amounts of Liability Components, Unamortized Discounts and Net Carrying Amounts for Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2016	Sep. 30, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Principal amount of liability components	$ 602,485		$ 610,314
Operating Partnership [Member]
Debt Instrument [Line Items]
Carrying amount of equity components	22,597		22,597
Unamortized debt issuance costs	(7,223)		(9,033)
Net carrying amount of liability components	602,485		610,314
Operating Partnership [Member] | Exchangeable Senior Notes 2.375% due 2033 (the 2013 Notes) [Member]
Debt Instrument [Line Items]
Carrying amount of equity components	0		0
Principal amount of liability components	49,384	$ 13,786	63,170
Unamortized discount - equity components	(475)		(1,187)
Unamortized cash discount - 2013 Notes	(110)	(183)	(281)		$ (3,750)
Unamortized debt issuance costs		$ (35)			$ (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
Principal amount of liability components	575,000		575,000
Unamortized discount - equity components	$ (14,091)		$ (17,355)
Unamortized debt issuance costs				$ (11,992)