Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Schedule of Available for Sale Securities by Type
Cost	$ 3,539	$ 3,144
Gross Unrealized Gains	35	35
Gross Unrealized Losses	(13)	(22)
Estimated Fair Value	3,561	3,157
U.S. States and Political Subdivisions Debt Securities [Member]
Schedule of Available for Sale Securities by Type
Cost	1,686	2,019
Gross Unrealized Gains	24	28
Gross Unrealized Losses	(4)	(11)
Estimated Fair Value	1,706	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	1,135	465
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(6)	(8)
Estimated Fair Value	1,136	460
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	19	19
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	19	19
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	87	92
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	0
Estimated Fair Value	90	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	549	486
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	(1)
Estimated Fair Value	549	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

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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