Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Interest Rate Caps Sold
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	$ 5,700	$ 8,000
Purchase [Member]
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	2,700	3,000
Interest Rate Cap [Member]
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	8,431	10,968
Interest Rate Cap [Member] | Subsidiary Issuers
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	3,000	5,000
Interest Rate Swap [Member]
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	1,500	1,000
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items] (Deprecated 2018-01-31)
Derivative, Notional Amount	$ 1,235	$ 934