Exchangeable Senior Notes - Summary of Repurchase of Debt (Detail) - Operating Partnership - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 21, 2013
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (5,407)	$ (6,620)
Exchangeable Senior Notes 2.375% due 2033 (the 2013 Notes)
Debt Instrument [Line Items]
Principal amount repurchased	37,709
Extinguishment of liability component	37,469
Reacquisition of equity component	21,000
Extinguishment of liability component	(37,469)
Discount on exchangeable senior notes	(230)		$ (3,750)
Unamortized debt issuance costs	(10)		$ (1,672)
Gain/(loss) on repurchase	0
Total consideration paid for repurchase	$ 58,469