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Commitments (Narrative) (Details) - Standby letters of credit [Member] - USD ($)
$ in Billions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Loss Contingencies [Line Items]    
Internal credit ratings (as a percentage of portfolio) - Pass 90.00% 91.00%
Internal credit ratings (as a percentage of portfolio) - Below Pass 10.00% 9.00%
Standby letters of credit - Terms outstanding - Minimum 1 year  
Standby letters of credit - Terms outstanding - Maximum 7 years  
Standby letters of credit - Assets securing certain specifically identified standby letters of credit $ 1.2  
Standby letters of credit and participations in standby letters of credit - Liability carrying amount $ 0.2