Goodwill Mortgage Servicing Rights (Commercial and Residential Mortgage Loan Servicing Assets - Key Valuation Assumptions) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 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	$ 1,082	$ 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	$ 477	$ 649	$ 681	$ 726
Weighted-average life (years)	4 years 1 month	4 years 1 month
Decline in fair value from 10% adverse change in prepayment rate	$ 8	$ 9
Decline in fair value from 20% adverse change in prepayment rate	16	17
Decline in fair value from 10% adverse change in interest rate	12	17
Decline in fair value from 20% adverse change in interest rate	24	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	$ 605	$ 995	$ 1,131	$ 1,257
Weighted-average life (years)	2 years 11 months	5 years 2 months
Decline in fair value from 10% adverse change in prepayment rate	$ 43	$ 46
Decline in fair value from 20% adverse change in prepayment rate	$ 81	$ 89
Spread over the benchmark curve	7.70%	7.69%
Decline in fair value from 10% adverse change in adjusted spread	$ 13	$ 27
Decline in fair value from 20% adverse change in adjusted spread	$ 26	$ 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	0.0451	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	0.2714	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	0.0757	0.0791