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Allowance for Losses on Financial Assets Allowances for Losses on Financial Assets (Narrative) (Details) (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Jan. 01, 2020
Allowance for Losses on Financial Assets [Line Items]    
Allowance for Credit Loss $ 374 $ 337
Accounting Standards Update 2016-13    
Allowance for Losses on Financial Assets [Line Items]    
Cumulative Effect of New Accounting Principle in Period of Adoption   (162)
Retained Earnings | Accounting Standards Update 2016-13    
Allowance for Losses on Financial Assets [Line Items]    
Cumulative Effect of New Accounting Principle in Period of Adoption   $ (162)