Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Interest Rate Caps Sold
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	$ 8,000	$ 7,400
Purchase [Member]
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	3,000	2,300
Interest Rate Cap [Member]
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	10,968	9,736
Interest Rate Cap [Member] | Subsidiary Issuers
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	5,000	5,100
Interest Rate Swap [Member]
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	1,000	1,950
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Derivative, Notional Amount	$ 934	$ 692