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Significant Credit Concentrations (Details) - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Maximum Credit Exposure by Category    
On-balance sheet $ 147 $ 150
Individuals [Member]    
Maximum Credit Exposure by Category    
On-balance sheet 98 104
Unused lines-of-credit 242 297
Financial Institutions [Member]    
Maximum Credit Exposure by Category    
On-balance sheet 28 25
United States Government And Agencies [Member]    
Maximum Credit Exposure by Category    
On-balance sheet 3 4
Other Concentration [Member]    
Maximum Credit Exposure by Category    
On-balance sheet $ 18 $ 17