Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available for Sale Securities by Type
Cost	$ 3,158	$ 3,144	$ 3,668
Gross Unrealized Gains	16	35	100
Gross Unrealized Losses	(15)	(22)	(9)
Estimated Fair Value	3,159	3,157	3,759
State and municipal obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	1,369	2,019	2,813
Gross Unrealized Gains	11	28	85
Gross Unrealized Losses	(3)	(11)	(5)
Estimated Fair Value	1,377	2,036	2,893
U.S. Government agency obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	11	12	2
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Estimated Fair Value	11	12	2
U.S. Government treasury obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	1,051	465	406
Gross Unrealized Gains	3	3	4
Gross Unrealized Losses	(9)	(8)	1
Estimated Fair Value	1,045	460	409
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	28	19	29
Gross Unrealized Gains	0	0	1
Gross Unrealized Losses	0	0	0
Estimated Fair Value	28	19	30
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	67	92	117
Gross Unrealized Gains	2	3	4
Gross Unrealized Losses	0	0	0
Estimated Fair Value	69	95	121
Equity securities [Member]
Schedule of Available for Sale Securities by Type
Cost	0	1	1
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Estimated Fair Value	0	1	1
Foreign government bonds and obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	581	486	250
Gross Unrealized Gains	0	1	6
Gross Unrealized Losses	0	(1)	1
Estimated Fair Value	581	486	255
Other [Member]
Schedule of Available for Sale Securities by Type
Cost	51	51	51
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(3)	(2)	(2)
Estimated Fair Value	$ 48	$ 49	$ 49