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Goodwill and Mortgage Servicing Rights (Commercial and Residential Mortgage Loan Servicing Assets - Key Valuation Assumptions) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]        
Fair Value $ 1,832 $ 1,758    
Mortgage Servicing Rights | Commercial Real Estate        
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]        
Fair Value $ 668 $ 576 $ 526 $ 506
Weighted-average life 4 years 4 months 24 days 4 years 7 months    
Constant prepayment rate (CPR) 8.51% 8.61%    
Decline in fair value from 10% adverse change in prepayment rate $ 12 $ 11    
Decline in fair value from 20% adverse change in prepayment rate $ 23 $ 21    
Discount rate 7.81% 7.52%    
Decline in fair value from 10% adverse change in interest rate $ 18 $ 16    
Decline in fair value from 20% adverse change in interest rate 36 31    
Mortgage Servicing Rights | Residential Mortgage        
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]        
Fair Value $ 1,164 $ 1,182 $ 1,063 $ 845
Weighted-average life 6 years 4 months 24 days 6 years 10 months    
Constant prepayment rate (CPR) 10.04% 9.41%    
Decline in fair value from 10% adverse change in prepayment rate $ 44 $ 45    
Decline in fair value from 20% adverse change in prepayment rate $ 85 $ 86    
Spread over the benchmark curve 8.30% 8.50%    
Decline in fair value from 10% adverse change in adjusted spread $ 35 $ 37    
Decline in fair value from 20% adverse change in adjusted spread $ 67 $ 72