Investment Securities (Details 1) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale investment securities with gross unrealized losses and length of time
Estimated Fair Value, Less than 12 months	$ 1,592	$ 807
Estimated Fair Value, 12 months or more	195	211
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(22)	(6)
Gross Unrealized Losses, 12 months or more	(9)	(8)
U.S. States and Political Subdivisions Debt Securities [Member]
Available-for-sale investment securities with gross unrealized losses and length of time
Estimated Fair Value, Less than 12 months	130	157
Estimated Fair Value, 12 months or more	22	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(2)	(3)
Gross Unrealized Losses, 12 months or more	(1)	0
Equity securities [Member]
Available-for-sale investment securities with gross unrealized losses and length of time
Estimated Fair Value, Less than 12 months	0	0
Estimated Fair Value, 12 months or more	0	36
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months		0
Gross Unrealized Losses, 12 months or more		(2)
U.S. Government treasury obligations [Member]
Available-for-sale investment securities with gross unrealized losses and length of time
Estimated Fair Value, Less than 12 months	1,462	650
Estimated Fair Value, 12 months or more	173	175
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(20)	(3)
Gross Unrealized Losses, 12 months or more	$ (8)	$ (6)