Investments - Net investment income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income:
Investment income, before expense	$ 914	$ 898	$ 2,655	$ 2,642
Investment expense	(70)	(55)	(201)	(154)
Net investment income	844	843	2,454	2,488
Valuation of equity investments	198	0	149	0
Fixed income securities
Net Investment Income:
Investment income, before expense	527	519	1,544	1,564
Equity securities
Net Investment Income:
Investment income, before expense	35	37	130	130
Mortgage loans
Net Investment Income:
Investment income, before expense	52	52	163	157
Limited partnership interests
Net Investment Income:
Investment income, before expense	210	223	563	596
Valuation of equity investments	75		182
Short-term investments
Net Investment Income:
Investment income, before expense	19	9	50	21
Other
Net Investment Income:
Investment income, before expense	71	$ 58	205	$ 174
EMA limited partnerships \| Limited partnership interests
Net Investment Income:
Investment income, before expense	$ 135		$ 381

Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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Amount of expenses related to the generation of investment income.

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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.

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