Debt - Debt Summary (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Debt	$ 11,934,355	$ 9,380,199
Credit Facilities [Member]
Debt Instrument [Line Items]
Debt	207,978
Senior Notes [Member]
Debt Instrument [Line Items]
Debt	6,622,440	6,076,920
Exchangeable Senior Notes [Member]
Debt Instrument [Line Items]
Debt		456,766
Secured Mortgage Debt [Member]
Debt Instrument [Line Items]
Debt	$ 785,480	$ 1,050,591
Debts Of Operating Partnership [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	3.00%	3.60%
Debt	$ 11,934,355	$ 9,380,199
Debts Of Operating Partnership [Member] | Credit Facilities [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.20%
Debt	$ 207,978
Debts Of Operating Partnership [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	3.30%	3.60%
Debt	$ 6,622,440	$ 6,076,920
Debts Of Operating Partnership [Member] | Exchangeable Senior Notes [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate		3.30%
Debt		$ 456,766
Debts Of Operating Partnership [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.40%	1.30%
Debt	$ 2,442,848	$ 572,730
Debts Of Operating Partnership [Member] | Other Debt [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	6.20%	6.20%
Debt	$ 15,648	$ 16,086
Debts Of Operating Partnership [Member] | Secured Mortgage Debt [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	6.40%	6.10%
Debt	$ 785,480	$ 1,050,591
Debts Of Operating Partnership [Member] | Secured Mortgage Debt of Consolidated Entities [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	2.70%	2.50%
Debt	$ 1,859,961	$ 1,207,106