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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Residential Mortgage [Member]
Total principal balance	$ 56,042		$ 56,042		$ 58,320
Delinquent loans	695		695		899
Net charge-offs	13	$ 24	25	$ 49
Commercial Mortgages [Member]
Total principal balance	51,042		51,042		49,116
Delinquent loans	279		279		$ 355
Net charge-offs	$ 22	$ 56	$ 52	$ 411