Change in Accumulated Depreciation and Amortization of Real Estate Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate and Accumulated Depreciation [Line Items]
Beginning Balance	$ 4,306	$ 3,834	$ 3,404
Depreciation and amortization	537	496	450
Dispositions and other	(75)	(24)	(20)
Ending Balance	$ 4,768	$ 4,306	$ 3,834