Investments (details) - Investment Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment disclosure details
Fixed maturities, amortized cost	$ 61,316	$ 59,650
Gross unrealized gains	1,556	1,369
Gross unrealized losses	178	504
Fixed maturities, at fair value	62,694	60,515
Equity securities, cost	440	504
Gross unrealized gains	20	236
Gross unrealized losses	7	8
Equity securities, at fair value	453	732
Continuous unrealized loss position less than 12 months, fair value	9,153	18,310
Continuous unrealized loss position less than 12 months, gross unrealized losses	68	482
Continuous unrealized loss position 12 months or longer, fair value	5,870	561
Continuous unrealized loss position 12 months or longer, gross unrealized losses	117	30
Continuous unrealized loss position, total, fair value	15,023	18,871
Continuous unrealized loss position, total, gross unrealized losses	185	512
Below investment grade [Member]
Investment disclosure details
Fixed maturities, at fair value	1,670	1,760
Fixed maturities [Member]
Investment disclosure details
Gross realized gains	42	79	$ 95
Gross realized losses	38	20	14
Continuous unrealized loss position less than 12 months, fair value	9,132	18,263
Continuous unrealized loss position less than 12 months, gross unrealized losses	68	480
Continuous unrealized loss position 12 months or longer, fair value	5,780	492
Continuous unrealized loss position 12 months or longer, gross unrealized losses	110	24
Continuous unrealized loss position, total, fair value	14,912	18,755
Continuous unrealized loss position, total, gross unrealized losses	178	504
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,080	2,031
Gross unrealized gains	4	9
Gross unrealized losses	8	5
Fixed maturities, at fair value	2,076	2,035
Continuous unrealized loss position less than 12 months, fair value	1,150	1,124
Continuous unrealized loss position less than 12 months, gross unrealized losses	5	5
Continuous unrealized loss position 12 months or longer, fair value	470
Continuous unrealized loss position 12 months or longer, gross unrealized losses	3
Continuous unrealized loss position, total, fair value	1,620	1,124
Continuous unrealized loss position, total, gross unrealized losses	8	5
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	29,996	31,550
Gross unrealized gains	968	712
Gross unrealized losses	49	352
Fixed maturities, at fair value	30,915	31,910
Continuous unrealized loss position less than 12 months, fair value	505	9,781
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	352
Continuous unrealized loss position 12 months or longer, fair value	2,959	12
Continuous unrealized loss position 12 months or longer, gross unrealized losses	47
Continuous unrealized loss position, total, fair value	3,464	9,793
Continuous unrealized loss position, total, gross unrealized losses	49	352
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,488	13,955
Gross unrealized gains	444	271
Gross unrealized losses	26	182
Fixed maturities, at fair value	13,906	14,044
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	11,307	10,910
Gross unrealized gains	338	215
Gross unrealized losses	19	147
Fixed maturities, at fair value	11,626	10,978
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,443	1,717
Gross unrealized gains	44	36
Gross unrealized losses	3	22
Fixed maturities, at fair value	1,484	1,731
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,758	4,968
Gross unrealized gains	142	190
Gross unrealized losses	1	1
Fixed maturities, at fair value	3,899	5,157
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,505	1,631
Gross unrealized gains	14	34
Gross unrealized losses	10	3
Fixed maturities, at fair value	1,509	1,662
Continuous unrealized loss position less than 12 months, fair value	394	360
Continuous unrealized loss position less than 12 months, gross unrealized losses	6	3
Continuous unrealized loss position 12 months or longer, fair value	111
Continuous unrealized loss position 12 months or longer, gross unrealized losses	4
Continuous unrealized loss position, total, fair value	505	360
Continuous unrealized loss position, total, gross unrealized losses	10	3
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,334	1,614
Gross unrealized gains	87	100
Gross unrealized losses	11	6
Fixed maturities, at fair value	2,410	1,708
Continuous unrealized loss position less than 12 months, fair value	1,021	528
Continuous unrealized loss position less than 12 months, gross unrealized losses	7	5
Continuous unrealized loss position 12 months or longer, fair value	250	43
Continuous unrealized loss position 12 months or longer, gross unrealized losses	4	1
Continuous unrealized loss position, total, fair value	1,271	571
Continuous unrealized loss position, total, gross unrealized losses	11	6
Residential mortgage-backed pass-through securities classified as available-for-sale, GNMA, FNMA, FHLMC and Canadian Government [Member]
Investment disclosure details
Fixed maturities, at fair value	804	563
Residential collateralized mortgage obligations [Member]
Investment disclosure details
Fixed maturities, at fair value	$ 1,610	$ 1,150
Percent guaranteed by or fully collateralized by securities issued by GNMA, FNMA or FHLMC	55.00%	51.00%
Non-guaranteed residential collateralized mortgage obligations [Member]
Investment disclosure details
Fixed maturities, at fair value	$ 717	$ 566
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	25,311	22,737
Gross unrealized gains	478	508
Gross unrealized losses	100	138
Fixed maturities, at fair value	25,689	23,107
Continuous unrealized loss position less than 12 months, fair value	6,062	6,470
Continuous unrealized loss position less than 12 months, gross unrealized losses	48	115
Continuous unrealized loss position 12 months or longer, fair value	1,990	437
Continuous unrealized loss position 12 months or longer, gross unrealized losses	52	23
Continuous unrealized loss position, total, fair value	8,052	6,907
Continuous unrealized loss position, total, gross unrealized losses	100	138
Commercial mortgage-backed securities [Member]
Investment disclosure details
Fixed maturities, at fair value	1,170	938
Commercial mortgage-backed securities, containing guarantees by the U.S. government or a government-sponsored enterprise [Member]
Investment disclosure details
Fixed maturities, at fair value	475	290
Commercial mortgage-backed securities, non-guaranteed securities [Member]
Investment disclosure details
Fixed maturities, at fair value	693	648
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	90	87
Gross unrealized gains	5	6
Fixed maturities, at fair value	95	93
Equity securities [Member]
Investment disclosure details
Gross realized gains	239	17	16
Gross realized losses	3	3	10
Continuous unrealized loss position less than 12 months, fair value	21	47
Continuous unrealized loss position less than 12 months, gross unrealized losses		2
Continuous unrealized loss position 12 months or longer, fair value	90	69
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7	6
Continuous unrealized loss position, total, fair value	111	116
Continuous unrealized loss position, total, gross unrealized losses	7	8
Public common stock [Member]
Investment disclosure details
Equity securities, cost	332	390
Gross unrealized gains	8	216
Gross unrealized losses	1	3
Equity securities, at fair value	339	603
Continuous unrealized loss position less than 12 months, fair value	18	45
Continuous unrealized loss position less than 12 months, gross unrealized losses		2
Continuous unrealized loss position 12 months or longer, fair value	34	10
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	1
Continuous unrealized loss position, total, fair value	52	55
Continuous unrealized loss position, total, gross unrealized losses	1	3
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	108	114
Gross unrealized gains	12	20
Gross unrealized losses	6	5
Equity securities, at fair value	114	129
Continuous unrealized loss position less than 12 months, fair value	3	2
Continuous unrealized loss position 12 months or longer, fair value	56	59
Continuous unrealized loss position 12 months or longer, gross unrealized losses	6	5
Continuous unrealized loss position, total, fair value	59	61
Continuous unrealized loss position, total, gross unrealized losses	6	5
Real estate [Member]
Investment disclosure details
Gross realized gains	10	7	4
Gross realized losses	$ 0	$ 0	$ 0