Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-Sale Debt Securities
Amortized Cost	$ 19,052	$ 24,216
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(3)	(53)
Available-for-sale debt securities	19,053	24,164
U.S. treasury securities
Available-for-Sale Debt Securities
Amortized Cost	2,860	3,978
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1)	(9)
Available-for-sale debt securities	2,860	3,969
Certificates of deposit
Available-for-Sale Debt Securities
Amortized Cost	4,152	4,361
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale debt securities	4,152	4,361
U.S. government agencies securities
Available-for-Sale Debt Securities
Amortized Cost	1,145	943
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(5)
Available-for-sale debt securities	1,145	938
Non-U.S. government securities
Available-for-Sale Debt Securities
Amortized Cost	192	307
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2)
Available-for-sale debt securities	192	305
Corporate debt securities
Available-for-Sale Debt Securities
Amortized Cost	10,472	13,095
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(1)	(29)
Available-for-sale debt securities	10,474	13,067
Residential mortgage and asset-backed securities
Available-for-Sale Debt Securities
Amortized Cost	231	1,532
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(8)
Available-for-sale debt securities	$ 230	$ 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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