Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Amortized Cost	$ 10,868	$ 10,814
Estimated Fair Value
Estimated Fair Value	11,136	11,301
Fixed Maturities [Member]
Amortized Cost
Due within one year	235
Due after one year through five years	1,456
Due after five years through 10 years	2,403
Due after 10 years	4,453
Amortized Cost	9,957	10,187
Estimated Fair Value
Due within one year	231
Due after one year through five years	1,458
Due after five years through 10 years	2,410
Due after 10 years	4,736
Estimated Fair Value	10,225	10,674
Fixed Maturities [Member] \| RMBS [Member]
Amortized Cost
Amortized Cost	857	852
Estimated Fair Value
Estimated Fair Value	844	861
Fixed Maturities [Member] \| CMBS [Member]
Amortized Cost
Amortized Cost	553	540
Estimated Fair Value
Estimated Fair Value	$ 546	$ 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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