Fair Value Disclosure of Financial Instruments - Additional Information (Detail) (USD $) In Billions, unless otherwise specified					9 Months Ended
	Sep. 30, 2014 Fixed Rate Debt	Dec. 31, 2013 Fixed Rate Debt	Sep. 30, 2014 Variable Rate Debt	Dec. 31, 2013 Variable Rate Debt	Sep. 30, 2014 Minimum [Member] Secured Debt [Member]	Sep. 30, 2014 Maximum [Member] Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	$ 2.49	$ 2.30	$ 0.88	$ 1.12
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	$ 2.46	$ 2.29	$ 0.97	$ 1.18
Debt Renewal Period					30 days	90 days