Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available for Sale Securities by Type
Cost	$ 3,652	$ 3,668
Gross Unrealized Gains	78	100
Gross Unrealized Losses	(2)	(9)
Estimated Fair Value	3,728	3,759
U.S. States and Political Subdivisions Debt Securities [Member]
Schedule of Available for Sale Securities by Type
Cost	2,255	2,813
Gross Unrealized Gains	50	85
Gross Unrealized Losses	0	(5)
Estimated Fair Value	2,305	2,893
U.S. Government agency obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	12	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	12	2
U.S. Government treasury obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	529	406
Gross Unrealized Gains	12	4
Gross Unrealized Losses	0	(1)
Estimated Fair Value	541	409
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	20	29
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	21	30
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	103	117
Gross Unrealized Gains	5	4
Gross Unrealized Losses	0	0
Estimated Fair Value	108	121
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	682	250
Gross Unrealized Gains	10	6
Gross Unrealized Losses	0	(1)
Estimated Fair Value	692	255
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