Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 24,216	$ 29,685
Gross Unrealized Gains	1	4
Gross Unrealized Losses	(53)	(102)
Available-for-sale debt securities	24,164	29,587
U.S. treasury securities
Available-for-Sale Securities
Amortized Cost	3,978	4,090
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(29)
Available-for-sale debt securities	3,969	4,061
Certificates of deposit
Available-for-Sale Securities
Amortized Cost	4,361	5,131
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale debt securities	4,361	5,131
U.S. government agencies securities
Available-for-Sale Securities
Amortized Cost	943	934
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(8)
Available-for-sale debt securities	938	926
Non-U.S. government securities
Available-for-Sale Securities
Amortized Cost	307	668
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(4)
Available-for-sale debt securities	305	664
Corporate debt securities
Available-for-Sale Securities
Amortized Cost	13,095	14,790
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(29)	(46)
Available-for-sale debt securities	13,067	14,747
Residential mortgage and asset-backed securities
Available-for-Sale Securities
Amortized Cost	1,532	4,072
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(8)	(15)
Available-for-sale debt securities	$ 1,524	$ 4,058

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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