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Commitments (Narrative) (Details) - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Standby letters of credit    
Loss Contingencies [Line Items]    
Internal credit ratings (as a percentage of portfolio) - Pass 91.00% 91.00%
Internal credit ratings (as a percentage of portfolio) - Criticized 9.00% 9.00%
Standby letters of credit - Assets securing certain specifically identified standby letters of credit $ 1.1  
Standby letters of credit and participations in standby letters of credit - Liability carrying amount $ 0.2  
Less than 1 year    
Loss Contingencies [Line Items]    
Standby letters of credit, term 1 year  
Maximum    
Loss Contingencies [Line Items]    
Standby letters of credit, term 6 years