Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-Sale Securities
Amortized Cost	$ 16,499	$ 24,216
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(1)	(53)
Available-for-sale debt securities	16,500	24,164
U.S. treasury securities
Available-for-Sale Securities
Amortized Cost	2,433	3,978
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(9)
Available-for-sale debt securities	2,433	3,969
Certificates of deposit
Available-for-Sale Securities
Amortized Cost	3,517	4,361
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale debt securities	3,517	4,361
U.S. government agencies securities
Available-for-Sale Securities
Amortized Cost	1,081	943
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(5)
Available-for-sale debt securities	1,081	938
Non-U.S. government securities
Available-for-Sale Securities
Amortized Cost	174	307
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2)
Available-for-sale debt securities	174	305
Corporate debt securities
Available-for-Sale Securities
Amortized Cost	9,203	13,095
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(1)	(29)
Available-for-sale debt securities	9,204	13,067
Residential mortgage and asset-backed securities
Available-for-Sale Securities
Amortized Cost	91	1,532
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(8)
Available-for-sale debt securities	$ 91	$ 1,524

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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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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