Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities by Type
Cost	$ 6,112	$ 3,158
Gross Unrealized Gains	8	16
Gross Unrealized Losses	(49)	(15)
Estimated Fair Value	6,071	3,159
U.S. States and Political Subdivisions Debt Securities [Member]
Schedule of Available for Sale Securities by Type
Cost	682	1,369
Gross Unrealized Gains	5	11
Gross Unrealized Losses	(3)	(3)
Estimated Fair Value	684	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	(1)	0
Estimated Fair Value	9	11
U.S. Government treasury obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	4,688	1,051
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(39)	(9)
Estimated Fair Value	4,651	1,045
Corporate debt securities [Member]
Schedule of Available for Sale Securities by Type
Cost	32	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	32	28
Mortgage-backed securities [Member]
Schedule of Available for Sale Securities by Type
Cost	54	67
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(1)	0
Estimated Fair Value	54	69
Equity securities [Member]
Schedule of Available for Sale Securities by Type
Cost	51	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(3)
Estimated Fair Value	47	48
Foreign government bonds and obligations [Member]
Schedule of Available for Sale Securities by Type
Cost	595	581
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	$ 594	$ 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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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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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