Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Amortized Cost	$ 10,744	$ 10,814
Estimated Fair Value
Estimated Fair Value	10,930	11,301
Fixed Maturities [Member]
Amortized Cost
Due within one year	229
Due after one year through five years	1,499
Due after five years through 10 years	2,426
Due after 10 years	4,348
Amortized Cost	10,006	10,187
Estimated Fair Value
Due within one year	229
Due after one year through five years	1,492
Due after five years through 10 years	2,411
Due after 10 years	4,584
Estimated Fair Value	10,192	10,674
Fixed Maturities [Member] \| RMBS [Member]
Amortized Cost
Amortized Cost	967	852
Estimated Fair Value
Estimated Fair Value	948	861
Fixed Maturities [Member] \| CMBS [Member]
Amortized Cost
Amortized Cost	537	540
Estimated Fair Value
Estimated Fair Value	$ 528	$ 549

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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