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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 22,662	$ 29,685
Gross Unrealized Gains	3	4
Gross Unrealized Losses	(104)	(102)
Fair Value	22,561	29,587
Corporate debt securities
Available-for-Sale Securities
Amortized Cost	12,144	14,790
Gross Unrealized Gains	3	3
Gross Unrealized Losses	(55)	(46)
Fair Value	12,092	14,747
U.S. treasury securities
Available-for-Sale Securities
Amortized Cost	3,412	4,090
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(21)	(29)
Fair Value	3,391	4,061
Residential mortgage and asset-backed securities
Available-for-Sale Securities
Amortized Cost	2,674	4,072
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(17)	(15)
Fair Value	2,657	4,058
U.S. government agencies securities
Available-for-Sale Securities
Amortized Cost	880	934
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(8)	(8)
Fair Value	872	926
Certificates of deposit
Available-for-Sale Securities
Amortized Cost	3,145	5,131
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	3,145	5,131
Non-U.S. government securities
Available-for-Sale Securities
Amortized Cost	407	668
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(4)
Fair Value	$ 404	$ 664

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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