Investments (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments disclosure
Fixed maturities, due in one year or less, amortized cost	$ 4,720
Fixed maturities, due after 1 year through 5 years, amortized cost	16,948
Fixed maturities, due after 5 years through 10 years, amortized cost	14,545
Fixed maturities, due after 10 years, amortized cost	22,769
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	58,982
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	2,334
Fixed maturities, amortized cost	61,316	$ 59,650
Fixed maturities, due in one year or less, fair value	4,749
Fixed maturities, due after 1 year through 5 years, fair value	17,283
Fixed maturities, due after 5 years through 10 years, fair value	14,859
Fixed maturities, due after 10 years, fair value	23,393
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	60,284
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	2,410
Fixed maturities, at fair value	62,694	60,515
Securities on loan as part of a tri-party lending agreement	304	286
Proceeds from sales of fixed maturities classified as available for sale	1,854	1,417	$ 1,950
Insurance subsidiaries' securities on deposit at financial institutions in certain states pursuant to the respective states' insurance regulatory requirements	4,410	4,560
Fair value of funds deposited with third parties to be used as collateral to secure various liabilities on behalf of insureds, cedants and other creditors	35	35
Fair value of other investments pledged as collateral securing outstanding letters of credit	1	3
Proceeds from sales of equity securities	765	92	59
Proceeds from sale of real estate investments	23	69	$ 31
Accumulated depreciation on real estate held for investment purposes	364	332
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2018	90
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2019	85
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2020	70
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2021	56
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2022	44
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2023 and thereafter	$ 58
Maximum original maturity of short-term securities	1 year
Combined average days to maturity of short-term securities (in days)	71 days
Amortized cost of short-term securities (which approximates fair value)	$ 4,900	$ 4,870
Minimum percentage of equity interest that must be held by the Company in order for the investment in a VIE to be considered significant	20.00%