Fair Value Disclosure of Financial Instruments - Additional Information (Detail) (USD $) In Billions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Fixed Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	2.55	$ 2.54
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	2.47	2.50
Variable Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	0.99	0.97
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	1.01	$ 1.02
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