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	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Principal amount of liability components	$ 570,425	$ 604,276
Operating Partnership
Debt Instrument [Line Items]
Carrying amount of equity components	22,597	22,597
Unamortized debt issuance costs	(5,407)	(6,620)
Net carrying amount of liability components	570,425	604,276
Operating Partnership | Exchangeable Senior Notes 2.375% due 2033 (the 2013 Notes)
Debt Instrument [Line Items]
Carrying amount of equity components	0	0
Principal amount of liability components	11,550	49,259
Unamortized discount - equity components	0	(315)
Unamortized cash discount - 2013 Notes	0	(74)
Unamortized debt issuance costs	(10)			$ (1,672)
Operating Partnership | Exchangeable Senior Notes 3.125% due 2035 (the 2015 Notes)
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	$ (10,718)	$ (12,974)
Unamortized debt issuance costs			$ (11,992)