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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 23,523	$ 29,685
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(72)	(102)
Fair Value	23,453	29,587
U.S. treasury securities
Available-for-Sale Securities
Amortized Cost	3,090	4,090
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15)	(29)
Fair Value	3,075	4,061
Certificates of deposit
Available-for-Sale Securities
Amortized Cost	4,392	5,131
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,392	5,131
U.S. government agencies securities
Available-for-Sale Securities
Amortized Cost	938	934
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6)	(8)
Fair Value	932	926
Non-U.S. government securities
Available-for-Sale Securities
Amortized Cost	262	668
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(4)
Fair Value	260	664
Corporate debt securities
Available-for-Sale Securities
Amortized Cost	12,792	14,790
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(37)	(46)
Fair Value	12,757	14,747
Residential mortgage and asset-backed securities
Available-for-Sale Securities
Amortized Cost	2,049	4,072
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(12)	(15)
Fair Value	$ 2,037	$ 4,058

X

- Definition

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).

+ References

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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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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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