Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net investment income
Investment income, before expense	$ 444	$ 493	$ 875	$ 918
Investment expense	(25)	(20)	(49)	(41)
Net investment income	419	473	826	877
Fixed income securities
Net investment income
Investment income, before expense	247	269	498	537
Mortgage loans
Net investment income
Investment income, before expense	53	45	97	94
Equity securities
Net investment income
Investment income, before expense	16	14	23	29
Limited partnership interests
Net investment income
Investment income, before expense	92	135	188	200
Net investment income	10		48
Short-term investments
Net investment income
Investment income, before expense	5	2	9	3
Policy loans
Net investment income
Investment income, before expense	8	7	15	15
Other
Net investment income
Investment income, before expense	23	$ 21	45	$ 40
Equity method limited partnerships \| Limited partnership interests
Net investment income
Net investment income	$ 82		$ 140

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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