Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available for Sale Securities by Type
Cost	$ 3,231	$ 3,144
Gross Unrealized Gains	28	35
Gross Unrealized Losses	(9)	(22)
Estimated Fair Value	3,250	3,157
U.S. States and Political Subdivisions Debt Securities [Member]
Schedule of Available for Sale Securities by Type
Cost	1,412	2,019
Gross Unrealized Gains	17	28
Gross Unrealized Losses	(2)	(11)
Estimated Fair Value	1,427	2,036
U.S. Government agency obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	12	12
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	12	12
U.S. Government treasury obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	1,115	465
Gross Unrealized Gains	8	3
Gross Unrealized Losses	(5)	(8)
Estimated Fair Value	1,118	460
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	28	19
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	28	19
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	74	92
Gross Unrealized Gains	2	3
Gross Unrealized Losses	0	0
Estimated Fair Value	76	95
Equity securities [Member]
Schedule of Available for Sale Securities by Type
Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1	1
Foreign government bonds and obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	539	486
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(1)
Estimated Fair Value	540	486
Availabe For Sale Securities Other [Member]
Schedule of Available for Sale Securities by Type
Cost	50	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(2)
Estimated Fair Value	$ 48	$ 48