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Allowance for Credit Losses and Reserve for Unfunded Lending Commitments - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit loss	$ 16,129		$ 7,208
Allowance for credit losses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (Decrease) in Allowance for Credit Loss	8,900
Allowance for credit loss	$ 16,129	$ 16,832	$ 7,208	$ 7,037	$ 7,133	$ 7,220

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Increase (Decrease) in Allowance for Credit Loss

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- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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