Investments (Net investment income) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Net Investment Income [Line Items]
Gross investment income	$ 587	$ 505
Investment expense	(16)	(15)
Net investment income	571	490
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	455	446
Equity securities
Net Investment Income [Line Items]
Gross investment income	30	10
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	76	30
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	12	11
Short term investments
Net Investment Income [Line Items]
Gross investment income	10	6
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	2	2
Other
Net Investment Income [Line Items]
Gross investment income	$ 2	$ 0

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
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Amount of expenses related to the generation of investment income.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 2
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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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/disclosureRef
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-Paragraph 1
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