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, 2018	Mar. 31, 2017	Dec. 31, 2017
Residential Mortgage [Member]
Total principal balance	$ 57,339		$ 58,320
Delinquent loans	796		899
Net charge-offs	12	$ 25
Commercial Mortgages [Member]
Total principal balance	47,480		49,116
Delinquent loans	298		$ 355
Net charge-offs	$ 30	$ 355