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
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Residential Mortgage [Member]
Total principal balance	$ 48,468		$ 49,323
Delinquent loans	446		492
Net charge-offs	8	$ 11
Commercial Mortgages [Member]
Total principal balance	41,514		42,414
Delinquent loans	10		$ 64
Net charge-offs	$ 99	$ 119