Goodwill Mortgage Servicing Rights (Commercial and Residential Mortgage Loan Servicing Assets - Key Valuation Assumptions) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
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	$ 1,483	$ 1,983
Mortgage Servicing Rights [Member] | Commercial Mortgage [Member]
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	$ 595	$ 726	$ 766	$ 668
Weighted-average life (years)	4 years	4 years 1 month 6 days
Decline in fair value from 10% adverse change in prepayment rate	$ 8	$ 10
Decline in fair value from 20% adverse change in prepayment rate	16	19
Decline in fair value from 10% adverse change in interest rate	15	19
Decline in fair value from 20% adverse change in interest rate	30	39
Mortgage Servicing Rights [Member] | Residential Mortgage [Member]
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	$ 888	$ 1,257	$ 1,370	$ 1,164
Weighted-average life (years)	4 years 3 months 18 days	6 years 10 months 24 days
Decline in fair value from 10% adverse change in prepayment rate	$ 46	$ 41
Decline in fair value from 20% adverse change in prepayment rate	$ 87	$ 79
Spread over the benchmark curve	7.73%	8.06%
Decline in fair value from 10% adverse change in adjusted spread	$ 22	$ 37
Decline in fair value from 20% adverse change in adjusted spread	$ 43	$ 73
Mortgage Servicing Rights [Member] | Measurement Input, Constant Prepayment Rate [Member] | Commercial Mortgage [Member]
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]
Servicing asset, measurement input	0.0460	0.0565
Mortgage Servicing Rights [Member] | Measurement Input, Constant Prepayment Rate [Member] | Residential Mortgage [Member]
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]
Servicing asset, measurement input	0.1744	0.0869
Mortgage Servicing Rights [Member] | Measurement Input, Discount Rate [Member] | Commercial Mortgage [Member]
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]
Servicing asset, measurement input	0.0788	0.0839