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Commitments (Narrative) (Details) - Standby letters of credit - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Internal credit ratings (as a percentage of portfolio) - Pass 91.00% 94.00%
Internal credit ratings (as a percentage of portfolio) - Below pass 9.00% 6.00%
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 $ 1.3  
Standby letters of credit and participations in standby letters of credit - Liability carrying amount $ 0.2