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Commitments (Narrative) (Details) - Standby letters of credit [Member] - USD ($)
$ in Billions
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Internal credit ratings (as a percentage of portfolio) - Pass 92.00% 94.00%
Standby letters of credit - Terms outstanding - Minimum 1 year  
Standby letters of credit - Terms outstanding - Maximum 8 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