Change in Accumulated Depreciation and Amortization of Real Estate Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate And Accumulated Depreciation Other Required Disclosures [Abstract]
Beginning Balance	$ 5,949	$ 5,666	$ 5,283
Depreciation and amortization	563	572	558
Dispositions and other	(247)	(159)	(148)
Depreciation on assets held for sale	7	(130)	(27)
Ending Balance	$ 6,272	$ 5,949	$ 5,666