Derivative Financial Instruments - Impact of Derivatives on Consolidated Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Assets:
Derivative assets	$ 375	$ 230
Derivative Liabilities:
Derivative liabilities	(713)	(664)
Net total derivatives	(338)	(434)
Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative Assets:
Derivative assets	369	226
Derivative Liabilities:
Derivative liabilities	(473)	(575)
Net total derivatives	(104)	(349)
Trading [Member]
Derivative Assets:
Derivative assets	6	4
Derivative Liabilities:
Derivative liabilities	(240)	(89)
Net total derivatives	(234)	(85)
Interest Rate Swaps [Member]
Derivative Assets:
Derivative assets	363	230
Derivative Liabilities:
Derivative liabilities	(15)	(20)
Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative Assets:
Derivative assets	357	226
Interest Rate Swaps [Member] | Trading [Member]
Derivative Assets:
Derivative assets	6	4
Derivative Liabilities:
Derivative liabilities	(15)	(20)
Cross-Currency Interest Rate Swaps [Member]
Derivative Assets:
Derivative assets	12
Derivative Liabilities:
Derivative liabilities	(473)	(575)
Cross-Currency Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative Assets:
Derivative assets	12
Derivative Liabilities:
Derivative liabilities	(473)	(575)
Floor Income Contracts [Member]
Derivative Liabilities:
Derivative liabilities	(225)	(68)
Floor Income Contracts [Member] | Trading [Member]
Derivative Liabilities:
Derivative liabilities	$ (225)	(68)
Other [Member]
Derivative Liabilities:
Derivative liabilities		(1)
Other [Member] | Trading [Member]
Derivative Liabilities:
Derivative liabilities		$ (1)