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Asset Quality - Changes in Liability for Credit Losses on Lending Related Commitments (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jan. 01, 2020
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]              
Balance at the end of the prior period $ 68 $ 197 $ 68 $ 68      
Liability for credit losses on contingent guarantees at the end of the prior period           $ 0 $ 7
Balance at beginning of period 68 178 161 68 $ 68 197 68
Provision (credit) for losses on off balance sheet exposures   (19) 20        
Balance at end of period   178 161 197 68    
Impact of ASC 326 Adoption              
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]              
Cumulative effect from changes in accounting principle       0 66    
Cumulative Effect, Period of Adoption, Adjusted Balance              
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]              
Balance at the end of the prior period 141 197 141 141      
Balance at beginning of period 141 $ 197 $ 141 141 141 $ 197 $ 141
Balance at end of period       $ 197 $ 141    
Other Financial Assets | Impact of ASC 326 Adoption              
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]              
Cumulative effect from changes in accounting principle $ 4