Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
AvailableForSaleSecuritiesAbstract
Cost	$ 6,448	$ 4,659
Gross Unrealized Gains	22	10
Gross Unrealized Losses	(13)	(22)
Estimated Fair Value	6,457	4,647
U.S. States and Political Subdivisions Debt Securities [Member]
AvailableForSaleSecuritiesAbstract
Cost	342	594
Gross Unrealized Gains	6	4
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	347	596
U.S. Government agency obligations [Member]
AvailableForSaleSecuritiesAbstract
Cost	10	10
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	10	10
U.S. Government treasury obligations [Member]
AvailableForSaleSecuritiesAbstract
Cost	5,404	3,452
Gross Unrealized Gains	14	5
Gross Unrealized Losses	(9)	(17)
Estimated Fair Value	5,409	3,440
Corporate debt securities [Member]
AvailableForSaleSecuritiesAbstract
Cost	29	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	29	28
Mortgage-backed securities [Member]
AvailableForSaleSecuritiesAbstract
Cost	48	50
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	49	51
Equity securities [Member]
AvailableForSaleSecuritiesAbstract
Cost	50	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(3)
Estimated Fair Value	47	48
Foreign government bonds and obligations [Member]
AvailableForSaleSecuritiesAbstract
Cost	565	474
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 566	$ 474

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

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- References

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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- Definition

Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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