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Summary of Significant Accounting Policies Newly Adopted Accounting Standards (Detail) - USD ($) $ in Millions | Mar. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019
--- | --- | --- | ---
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Allowance for credit loss | $ 14,073 |  |
Retained earnings | 36,552 |  | $ 40,340
Accounting Standards Update 2016-13 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption, Tax |  | $ 694 |
Retained earnings |  | 2,200 |
Adjustment amount |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Retained earnings |  | 11 |
Allowance for credit losses |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Allowance for credit loss | $ 14,073 |  |
Finance charge and fee reserve reclassification |  | 462 |
Allowance for credit losses | Accounting Standards Update 2016-13 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Allowance for credit loss |  | 2,900 |
Allowance for credit losses | Adjustment amount |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Allowance for credit loss |  |  | $ 2,845
Finance charge and fee reserve reclassification |  | $ 462 |

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