Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities by Type
Cost	$ 3,404	$ 3,158
Gross Unrealized Gains	14	16
Gross Unrealized Losses	(30)	(15)
Estimated Fair Value	3,388	3,159
U.S. States and Political Subdivisions Debt Securities [Member]
Schedule of Available for Sale Securities by Type
Cost	1,122	1,369
Gross Unrealized Gains	8	11
Gross Unrealized Losses	(5)	(3)
Estimated Fair Value	1,125	1,377
U.S. Government agency obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	10	11
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	10	11
U.S. Government treasury obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	1,678	1,051
Gross Unrealized Gains	4	3
Gross Unrealized Losses	(21)	(9)
Estimated Fair Value	1,661	1,045
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	29	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	29	28
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	63	67
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(1)	0
Estimated Fair Value	64	69
Equity securities [Member]
Schedule of Available for Sale Securities by Type
Cost	51	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(3)
Estimated Fair Value	48	48
Foreign government bonds and obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	451	581
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 451	$ 581

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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-SubTopic 10
-Section 45
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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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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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