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, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
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	$ 1,113	$ 1,644
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	$ 515	$ 649	$ 595	$ 726
Weighted-average life (years)	4 years 4 months	4 years 1 month
Decline in fair value from 10% adverse change in prepayment rate	$ 9	$ 9
Decline in fair value from 20% adverse change in prepayment rate	17	17
Decline in fair value from 10% adverse change in interest rate	14	17
Decline in fair value from 20% adverse change in interest rate	27	34
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	$ 598	$ 995	$ 888	$ 1,257
Weighted-average life (years)	3 years 2 months	5 years 2 months
Decline in fair value from 10% adverse change in prepayment rate	$ 40	$ 46
Decline in fair value from 20% adverse change in prepayment rate	$ 78	$ 89
Spread over the benchmark curve	9.27%	7.69%
Decline in fair value from 10% adverse change in adjusted spread	$ 16	$ 27
Decline in fair value from 20% adverse change in adjusted spread	$ 30	$ 52
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	0.0470	0.0456
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	0.2482	0.1351
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	0.0737	0.0791