Investments - Net investment income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Gross investment income	$ 524	$ 538	$ 1,573	$ 1,504
Investment expense	(15)	(14)	(44)	(43)
Net investment income	509	524	1,529	1,461
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	455	457	1,367	1,352
Equity securities
Net Investment Income [Line Items]
Gross investment income	1	1	4	8
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	51	65	157	97
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	9	8	24	30
Short term investments
Net Investment Income [Line Items]
Gross investment income	4	2	10	6
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	3	1	9	7
Other
Net Investment Income [Line Items]
Gross investment income	$ 1	$ 4	$ 2	$ 4

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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