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Allowance for Credit Losses - Change in Liability for Off-balance Sheet Credit Exposures (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]    
Balance at beginning of period $ 237 $ 158
Provision charged (credited) to income (82) 213
Balance at end of period 155  
Accounting Standards Update 2016-13 [Member]    
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]    
Balance at beginning of period [1] 0 54
As Previously Reported [Member]    
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]    
Balance at beginning of period $ 237 104
Balance at end of period   $ 371
[1] See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," in our 2020 Form 10-K for additional discussion