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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Residential Mortgage
Total principal balance	$ 58,320	$ 66,081
Delinquent loans	899	1,422
Net charge-offs	78	97
Commercial Mortgages
Total principal balance	49,116	45,855
Delinquent loans	355	941
Net charge-offs	$ 965	$ 1,439