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Summary of Significant Accounting Policies Newly Adopted Accounting Standards (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | --- | --- | ---
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Allowance for credit loss | $ 15,564 |  | $ 7,208 |  |
Retained earnings | 40,088 |  | 40,340 |  |
Accounting Standards Update 2016-13 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Deferred tax assets |  | $ 694 |  |  |
Retained earnings |  | 2,200 |  |  |
Allowance for credit losses |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Allowance for credit loss | $ 15,564 |  | $ 7,208 | $ 7,220 | $ 7,502
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 |  |  |

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