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
Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total amount of interest expense presented in the Consolidated Statement of Income	$ 1,092	$ 623
Asset and Liability Management Positions [Member] \| Fair Value Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to fair value hedged item	21	43
Asset and Liability Management Positions [Member] \| Fair Value Hedges [Member] \| Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to fair value hedge	(21)	(43)
Asset and Liability Management Positions [Member] \| Cash Flow Hedges [Member] \| Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gains (Losses) Recognized in Earnings related to cash flow hedge	$ (8)	$ 3

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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The amount of net gains or losses on interest rate cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

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