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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities in a Continuous Unrealized Loss Position Deemed not to be Other-than-Temporarily Impaired (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	$ (8)	$ (22)
Less than 12 Months, Fair Value	4,669	11,149
12 Months or Greater, Unrealized Losses	(64)	(80)
12 Months or Greater, Fair Value	7,361	9,168
Gross Unrealized Losses	(72)	(102)
Total Estimated Fair Value	12,030	20,317
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	(1)	(2)
Less than 12 Months, Fair Value	1,339	821
12 Months or Greater, Unrealized Losses	(14)	(27)
12 Months or Greater, Fair Value	1,613	3,240
Gross Unrealized Losses	(15)	(29)
Total Estimated Fair Value	2,952	4,061
U.S. government agencies securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(1)
Less than 12 Months, Fair Value	285	206
12 Months or Greater, Unrealized Losses	(6)	(7)
12 Months or Greater, Fair Value	612	700
Gross Unrealized Losses	(6)	(8)
Total Estimated Fair Value	897	906
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(1)
Less than 12 Months, Fair Value	0	203
12 Months or Greater, Unrealized Losses	(2)	(3)
12 Months or Greater, Fair Value	234	461
Gross Unrealized Losses	(2)	(4)
Total Estimated Fair Value	234	664
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	(6)	(14)
Less than 12 Months, Fair Value	2,636	7,674
12 Months or Greater, Unrealized Losses	(31)	(32)
12 Months or Greater, Fair Value	3,602	3,561
Gross Unrealized Losses	(37)	(46)
Total Estimated Fair Value	6,238	11,235
Residential mortgage and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	(1)	(4)
Less than 12 Months, Fair Value	409	2,245
12 Months or Greater, Unrealized Losses	(11)	(11)
12 Months or Greater, Fair Value	1,300	1,206
Gross Unrealized Losses	(12)	(15)
Total Estimated Fair Value	$ 1,709	$ 3,451

X

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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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), in continuous unrealized loss position for less than 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

+ References

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-Publisher FASB
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-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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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), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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