Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 24,788	$ 29,685
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(136)	(102)
Fair Value	24,654	29,587
Corporate debt securities
Available-for-Sale Securities
Amortized Cost	12,044	14,790
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(74)	(46)
Fair Value	11,972	14,747
U.S. treasury securities
Available-for-Sale Securities
Amortized Cost	3,912	4,090
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(28)	(29)
Fair Value	3,884	4,061
Residential mortgage and asset-backed securities
Available-for-Sale Securities
Amortized Cost	3,427	4,072
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(21)	(15)
Fair Value	3,406	4,058
U.S. government agencies securities
Available-for-Sale Securities
Amortized Cost	934	934
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(8)
Fair Value	925	926
Certificates of deposit
Available-for-Sale Securities
Amortized Cost	3,908	5,131
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	3,908	5,131
Non-U.S. government securities
Available-for-Sale Securities
Amortized Cost	563	668
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(4)
Fair Value	$ 559	$ 664

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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-Section 45
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-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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