Investment Securities (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Fair Value, Less than 12 months	$ 1,178	$ 808
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	26	5
Held-to-maturity Securities, Fair value Less 12 months or longer	672	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	46	32
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	488	471
Available-for-sale Securities, Unrealized Loss Less than 12 months	(16)	(8)
Available-for-sale Securities, Fair value 12 months or longer	1,282	1,305
Available-for-sale Securities, Unrealized Loss 12 months or longer	(70)	(46)
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	436	242
Available-for-sale Securities, Unrealized Loss Less than 12 months	(9)	(2)
Available-for-sale Securities, Fair value 12 months or longer	176	183
Available-for-sale Securities, Unrealized Loss 12 months or longer	(10)	(5)
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	82	80
Available-for-sale Securities, Unrealized Loss Less than 12 months	(2)	(1)
Available-for-sale Securities, Fair value 12 months or longer	4	4
Available-for-sale Securities, Unrealized Loss 12 months or longer	0	0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	8,951	4,066
Available-for-sale Securities, Unrealized Loss Less than 12 months	(183)	(19)
Available-for-sale Securities, Fair value 12 months or longer	5,395	5,671
Available-for-sale Securities, Unrealized Loss 12 months or longer	(254)	(137)
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	1,439	857
Available-for-sale Securities, Unrealized Loss Less than 12 months	(29)	(2)
Available-for-sale Securities, Fair value 12 months or longer	734	773
Available-for-sale Securities, Unrealized Loss 12 months or longer	(46)	(32)
Held-to-maturity Securities, Fair Value, Less than 12 months	1,147	773
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	26	5
Held-to-maturity Securities, Fair value Less 12 months or longer	672	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	46	32
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	71	76
Available-for-sale Securities, Unrealized Loss Less than 12 months	(2)	(1)
Available-for-sale Securities, Fair value 12 months or longer	21	21
Available-for-sale Securities, Unrealized Loss 12 months or longer	(1)	0
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	426	220
Available-for-sale Securities, Unrealized Loss Less than 12 months	(2)	(1)
Available-for-sale Securities, Fair value 12 months or longer	76	91
Available-for-sale Securities, Unrealized Loss 12 months or longer	(1)	0
Held-to-maturity Securities, Fair Value, Less than 12 months	31	35
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	0
Held-to-maturity Securities, Fair value Less 12 months or longer	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	932	529
Available-for-sale Securities, Unrealized Loss Less than 12 months	(20)	(4)
Available-for-sale Securities, Fair value 12 months or longer	191	194
Available-for-sale Securities, Unrealized Loss 12 months or longer	(14)	(7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	12,825	6,541
Available-for-sale Securities, Unrealized Loss Less than 12 months	(263)	(38)
Available-for-sale Securities, Fair value 12 months or longer	7,879	8,242
Available-for-sale Securities, Unrealized Loss 12 months or longer	$ (396)	$ (227)