Investments (Net investment income) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Gross investment income	$ 2,093	$ 2,047	$ 1,903
Investment expense	(59)	(59)	(63)
Net investment income	2,034	1,988	1,840
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	1,812	1,819	1,751
Equity securities
Net Investment Income [Line Items]
Gross investment income	12	10	12
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	207	155	92
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	34	41	33
Short term investments
Net Investment Income [Line Items]
Gross investment income	15	8	6
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	12	10	8
Other
Net Investment Income [Line Items]
Gross investment income	$ 1	$ 4	$ 1

Investment Income Including Realized Gains Losses on Trading Securities, Before Expenses

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- Definition

The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.

+ References

No definition available.

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- Definition

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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