Investments (details) - Investment Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment disclosure details
Fixed maturities, amortized cost	$ 58,878	$ 60,801
Gross unrealized gains	2,100	2,791
Gross unrealized losses	320	118
Fixed maturities, at fair value	60,658	63,474
Equity securities, cost	528	579
Gross unrealized gains	190	326
Gross unrealized losses	13	6
Equity securities, at fair value	705	899
Continuous unrealized loss position less than 12 months, fair value	11,221	2,744
Continuous unrealized loss position less than 12 months, gross unrealized losses	233	46
Continuous unrealized loss position 12 months or longer, fair value	1,008	3,536
Continuous unrealized loss position 12 months or longer, gross unrealized losses	100	78
Continuous unrealized loss position, total, fair value	12,229	6,280
Continuous unrealized loss position, total, gross unrealized losses	333	124
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	54
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	18
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	7
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	$ 85
Maxiumum [Member]
Investment disclosure details
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of the combined fixed maturity and equity security portfolios on a pretax basis	1.00%
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of shareholders' equity on an after-tax basis	1.00%
Fixed maturities [Member]
Investment disclosure details
Gross realized gains	$ 95	44	$ 66
Gross realized losses	14	12	25
Continuous unrealized loss position less than 12 months, fair value	11,126	2,619
Continuous unrealized loss position less than 12 months, gross unrealized losses	224	41
Continuous unrealized loss position 12 months or longer, fair value	937	3,493
Continuous unrealized loss position 12 months or longer, gross unrealized losses	96	77
Continuous unrealized loss position, total, fair value	12,063	6,112
Continuous unrealized loss position, total, gross unrealized losses	320	118
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	51
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	17
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	7
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	81
Fixed maturities [Member] | Below investment grade [Member]
Investment disclosure details
Fixed maturities, at fair value	1,710	1,910
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,202	2,022
Gross unrealized gains	8	36
Gross unrealized losses	16	5
Fixed maturities, at fair value	2,194	2,053
Continuous unrealized loss position less than 12 months, fair value	1,820	180
Continuous unrealized loss position less than 12 months, gross unrealized losses	15	2
Continuous unrealized loss position 12 months or longer, fair value	28	125
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	3
Continuous unrealized loss position, total, fair value	1,848	305
Continuous unrealized loss position, total, gross unrealized losses	16	5
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	30,027	31,895
Gross unrealized gains	1,393	1,688
Gross unrealized losses	9	10
Fixed maturities, at fair value	31,411	33,573
Continuous unrealized loss position less than 12 months, fair value	928	173
Continuous unrealized loss position less than 12 months, gross unrealized losses	7	1
Continuous unrealized loss position 12 months or longer, fair value	142	797
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	9
Continuous unrealized loss position, total, fair value	1,070	970
Continuous unrealized loss position, total, gross unrealized losses	9	10
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	12,744	12,366
Gross unrealized gains	577	644
Gross unrealized losses	3	5
Fixed maturities, at fair value	13,318	13,005
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,492	9,833
Gross unrealized gains	472	575
Gross unrealized losses	4	4
Fixed maturities, at fair value	9,960	10,404
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,978	2,467
Gross unrealized gains	97	137
Gross unrealized losses	2	1
Fixed maturities, at fair value	2,073	2,603
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	5,813	7,229
Gross unrealized gains	247	332
Fixed maturities, at fair value	6,060	7,561
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,829	2,320
Gross unrealized gains	45	48
Gross unrealized losses	1
Fixed maturities, at fair value	1,873	2,368
Continuous unrealized loss position less than 12 months, fair value	172	50
Continuous unrealized loss position less than 12 months, gross unrealized losses	1
Continuous unrealized loss position 12 months or longer, fair value		24
Continuous unrealized loss position, total, fair value	172	74
Continuous unrealized loss position, total, gross unrealized losses	1
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,863	2,052
Gross unrealized gains	124	165
Gross unrealized losses	6	4
Fixed maturities, at fair value	1,981	2,213
Continuous unrealized loss position less than 12 months, fair value	473	68
Continuous unrealized loss position less than 12 months, gross unrealized losses	4
Continuous unrealized loss position 12 months or longer, fair value	57	192
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	4
Continuous unrealized loss position, total, fair value	530	260
Continuous unrealized loss position, total, gross unrealized losses	6	4
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	676	872
Residential collateralized mortgage obligations [Member]
Investment disclosure details
Fixed maturities, at fair value	$ 1,300	$ 1,340
Percent guaranteed by or fully collateralized by securities issued by GNMA, FNMA or FHLMC	48.00%	46.00%
Non-guaranteed residential collateralized mortgage obligations [Member]
Investment disclosure details
Fixed maturities, at fair value	$ 683	$ 725
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	22,854	22,390
Gross unrealized gains	523	844
Gross unrealized losses	288	99
Fixed maturities, at fair value	23,089	23,135
Continuous unrealized loss position less than 12 months, fair value	7,725	2,148
Continuous unrealized loss position less than 12 months, gross unrealized losses	197	38
Continuous unrealized loss position 12 months or longer, fair value	710	2,355
Continuous unrealized loss position 12 months or longer, gross unrealized losses	91	61
Continuous unrealized loss position, total, fair value	8,435	4,503
Continuous unrealized loss position, total, gross unrealized losses	288	99
Commercial mortgage-backed securities [Member]
Investment disclosure details
Fixed maturities, at fair value	865	715
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	303	202
Commercial mortgage-backed securities, non-guaranteed securities [Member]
Investment disclosure details
Fixed maturities, at fair value	562	513
Other fixed maturities [Member]
Investment disclosure details
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	51
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	17
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	7
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	81
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	103	122
Gross unrealized gains	7	10
Fixed maturities, at fair value	110	132
Continuous unrealized loss position less than 12 months, fair value	8
Continuous unrealized loss position, total, fair value	8
Equity securities [Member]
Investment disclosure details
Gross realized gains	16	27	16
Gross realized losses	10	3	1
Continuous unrealized loss position less than 12 months, fair value	95	125
Continuous unrealized loss position less than 12 months, gross unrealized losses	9	5
Continuous unrealized loss position 12 months or longer, fair value	71	43
Continuous unrealized loss position 12 months or longer, gross unrealized losses	4	1
Continuous unrealized loss position, total, fair value	166	168
Continuous unrealized loss position, total, gross unrealized losses	13	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
Public common stock [Member]
Investment disclosure details
Equity securities, cost	386	400
Gross unrealized gains	164	295
Gross unrealized losses	7	4
Equity securities, at fair value	543	691
Continuous unrealized loss position less than 12 months, fair value	48	81
Continuous unrealized loss position less than 12 months, gross unrealized losses	6	4
Continuous unrealized loss position 12 months or longer, fair value	33	1
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	81	82
Continuous unrealized loss position, total, gross unrealized losses	7	4
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	142	179
Gross unrealized gains	26	31
Gross unrealized losses	6	2
Equity securities, at fair value	162	208
Continuous unrealized loss position less than 12 months, fair value	47	44
Continuous unrealized loss position less than 12 months, gross unrealized losses	3	1
Continuous unrealized loss position 12 months or longer, fair value	38	42
Continuous unrealized loss position 12 months or longer, gross unrealized losses	3	1
Continuous unrealized loss position, total, fair value	85	86
Continuous unrealized loss position, total, gross unrealized losses	6	2
Real estate [Member]
Investment disclosure details
Gross realized gains	4	6	7
Gross realized losses	$ 0	$ 0	$ 0