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, 2019	Mar. 31, 2018	Dec. 31, 2018
Residential Mortgage [Member]
Total principal balance	$ 53,055		$ 54,028
Delinquent loans	599		622
Net charge-offs	11	$ 12
Commercial Mortgages [Member]
Total principal balance	46,767		47,969
Delinquent loans	131		$ 234
Net charge-offs	$ 119	$ 30