Investment Securities (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	$ 485	$ 471
Available-for-sale Securities, Unrealized Loss Less than 12 months	(19)	(8)
Available-for-sale Securities, Fair value 12 months or longer	1,275	1,305
Available-for-sale Securities, Unrealized Loss 12 months or longer	(71)	(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	420	242
Available-for-sale Securities, Unrealized Loss Less than 12 months	(8)	(2)
Available-for-sale Securities, Fair value 12 months or longer	181	183
Available-for-sale Securities, Unrealized Loss 12 months or longer	(11)	(5)
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	56	80
Available-for-sale Securities, Unrealized Loss Less than 12 months	(1)	(1)
Available-for-sale Securities, Fair value 12 months or longer	25	4
Available-for-sale Securities, Unrealized Loss 12 months or longer	(1)	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	8,834	4,066
Available-for-sale Securities, Unrealized Loss Less than 12 months	(215)	(19)
Available-for-sale Securities, Fair value 12 months or longer	5,331	5,671
Available-for-sale Securities, Unrealized Loss 12 months or longer	(282)	(137)
Held-to-maturity Securities, Fair Value, Less than 12 months	1,152	773
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	33	5
Held-to-maturity Securities, Fair value Less 12 months or longer	668	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	50	32
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	1,642	857
Available-for-sale Securities, Unrealized Loss Less than 12 months	(35)	(2)
Available-for-sale Securities, Fair value 12 months or longer	688	773
Available-for-sale Securities, Unrealized Loss 12 months or longer	(58)	(32)
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	91	76
Available-for-sale Securities, Unrealized Loss Less than 12 months	(2)	(1)
Available-for-sale Securities, Fair value 12 months or longer	18	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	479	220
Available-for-sale Securities, Unrealized Loss Less than 12 months	(3)	(1)
Available-for-sale Securities, Fair value 12 months or longer	71	91
Available-for-sale Securities, Unrealized Loss 12 months or longer	(1)	0
Held-to-maturity Securities, Fair Value, Less than 12 months	24	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	885	529
Available-for-sale Securities, Unrealized Loss Less than 12 months	(26)	(4)
Available-for-sale Securities, Fair value 12 months or longer	254	194
Available-for-sale Securities, Unrealized Loss 12 months or longer	(17)	(7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Fair value Less than 12 months	12,892	6,541
Available-for-sale Securities, Unrealized Loss Less than 12 months	(309)	(38)
Available-for-sale Securities, Fair value 12 months or longer	7,843	8,242
Available-for-sale Securities, Unrealized Loss 12 months or longer	(442)	(227)
Held-to-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Fair Value, Less than 12 months	1,176	808
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	33	5
Held-to-maturity Securities, Fair value Less 12 months or longer	668	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 50	$ 32