Derivative Financial Instruments - Mark-to-Market Impact of Derivatives on Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	$ (32)	$ (40)	$ (25)	$ 8
Interest Expense and Derivative Instruments Hedging Activities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	(25)	(40)	(16)	8
Designated as Hedging Instrument [Member] \| Fair Value [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	7	(31)	9	(47)
Designated as Hedging Instrument [Member] \| Fair Value [Member] \| Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in net income on derivatives	186	(71)	290	(258)
Gains (losses) recognized in net income on hedged items	(200)	66	(317)	290
Net fair value hedge ineffectiveness gains (losses)	(14)	(5)	(27)	32
Designated as Hedging Instrument [Member] \| Fair Value [Member] \| Cross-Currency Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in net income on derivatives	41	(283)	29	(207)
Gains (losses) recognized in net income on hedged items	(20)	257	7	128
Net fair value hedge ineffectiveness gains (losses)	21	(26)	36	(79)
Trading [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	(32)	(9)	(25)	55
Trading [Member] \| Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	16	(4)	23	18
Trading [Member] \| Cross-Currency Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	1	(14)	(2)	2
Trading [Member] \| Floor Income Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized	$ (49)	10	(48)	33
Trading [Member] \| Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Mark-to-market gains (losses) recognized		$ (1)	$ 2	$ 2

Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
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- Definition

Amount of gain (loss) from the ineffectiveness of the hedge.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

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