Derivative Instruments - Effect of Fair Value and Cash Flow Hedge Accounting Included in Interest Expense on Consolidated Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Total amount of interest expense presented in the Consolidated Statement of Income	$ 346	$ 1,156	$ 1,711	$ 3,394
Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total amount of interest expense presented in the Consolidated Statement of Income	346	1,156	1,711	3,394
Asset and Liability Management Positions [Member] \| Fair Value Hedges [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to fair value hedged item	(27)	181	167	232
Asset and Liability Management Positions [Member] \| Fair Value Hedges [Member] \| Interest Rate Contracts [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to fair value hedge	28	(183)	(166)	(234)
Asset and Liability Management Positions [Member] \| Cash Flow Hedges [Member] \| Interest Rate Contracts [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to cash flow hedge	$ 3	$ 6	$ 7	$ (8)

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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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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Amount of the cost of borrowed funds accounted for as interest expense.

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