Fair Value Disclosure of Financial Instruments - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Preferred Stock, Redemption Price Per Share	$ 50.00
Fixed Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	$ 3,410	$ 3,130
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	3,230	3,000
Variable Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, fair value	1,300	1,500
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	$ 1,300	$ 1,500
Maximum [Member] | Conventional Variable Rate Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Renewal Period	90 days
Minimum [Member] | Conventional Variable Rate Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Renewal Period	30 days