Investments (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Gross Investment Income, Operating	$ 2,700	$ 2,599	$ 2,620
Less: Investment expenses	160	115	103
Net investment income	2,712	2,615	2,669
Securities Investment
Net Investment Income [Line Items]
Gross Investment Income, Operating	2,860	2,714	2,723
Fixed maturity securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	2,167	2,010	1,954
Equity securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	18	18	17
Mortgage loans
Net Investment Income [Line Items]
Gross Investment Income, Operating	384	360	337
Policy loans
Net Investment Income [Line Items]
Gross Investment Income, Operating	54	54	59
Real estate and real estate joint ventures
Net Investment Income [Line Items]
Gross Investment Income, Operating	32	108	80
Other limited partnership interests
Net Investment Income [Line Items]
Gross Investment Income, Operating	163	134	266
Cash, cash equivalents and short-term investments
Net Investment Income [Line Items]
Gross Investment Income, Operating	18	8	5
Operating joint venture
Net Investment Income [Line Items]
Gross Investment Income, Operating	11	11	2
Other
Net Investment Income [Line Items]
Gross Investment Income, Operating	13	11	3
FVO CSEs — interest income — commercial mortgage loans \| Variable Interest Entity, Primary Beneficiary, Consolidated Securitization Entities
Net Investment Income [Line Items]
Gross Investment Income, Operating	$ 12	$ 16	$ 49

Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16(Column G))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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+ References

No definition available.

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