Fair Value Disclosure of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fixed Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	$ 2,480	$ 2,540
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	2,450	2,500
Variable Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	970	970
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	$ 990	$ 1,020
Minimum [Member] | Conventional Variable Rate Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Renewal Period	30 days
Maximum [Member] | Conventional Variable Rate Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Renewal Period	90 days