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	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Principal amount of liability components	$ 568,644	$ 604,276
Operating Partnership [Member]
Debt Instrument [Line Items]
Carrying amount of equity components	22,597	22,597
Unamortized debt issuance costs	(6,008)	(6,620)
Net carrying amount of liability components	568,644	604,276
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	11,555	49,259
Unamortized discount - equity components	(37)	(315)
Unamortized cash discount - 2013 Notes	(9)	(74)
Unamortized debt issuance costs	(10)			$ (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	$ (11,857)	$ (12,974)
Unamortized debt issuance costs			$ (11,992)