Loan Sale and Servicing Activities and Variable Interest Entities (Cash Flows Associated with Loan Sale and Servicing Activities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Residential Mortgage [Member]
Carrying value of mortgage-backed securities held	$ 7,600	$ 6,700	$ 7,600	$ 6,700	$ 6,900
Cash flows from sales of loans	1,468	1,950	4,385	4,796
Cash flows from repurchases of previously transferred loans	103	133	331	396
Cash flows from servicing fees	95	95	281	281
Cash flows from servicing advances recovered		13	80	89
Cash flows from servicing advances (funded)	(4)
Cash flows on mortgage-backed securities held	372	466	1,066	1,235
Commercial Mortgages [Member]
Carrying value of mortgage-backed securities held	700	1,000	700	1,000	$ 900
Cash flows from sales of loans	1,280	1,342	3,639	2,796
Cash flows from servicing fees	32	31	95	93
Cash flows from servicing advances recovered			25
Cash flows from servicing advances (funded)	(1)	(7)
Cash flows on mortgage-backed securities held	$ 13	$ 31	$ 196	$ 228