Loan Sale and Servicing Activities and Variable Interest Entities (Principal Balance, Delinquent Loans (Loans 90 Days or More Past Due), and Net Charge-Offs Related to Serviced Loans) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Residential Mortgage [Member]
Total principal balance	$ 45,795		$ 45,795		$ 49,323
Delinquent loans	460		460		492
Net charge-offs	4	$ 8	14	$ 32
Commercial Mortgages [Member]
Total principal balance	41,379		41,379		42,414
Delinquent loans	311		311		$ 64
Net charge-offs	$ 4	$ 52	$ 103	$ 348