Investments (details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investment disclosure details
Fixed maturities, amortized cost	$ 61,995	$ 62,196
Fixed maturities, at fair value	64,271	63,956
Equity securities, cost	660	686
Equity securities, at fair value	938	943
Continuous unrealized loss position less than 12 months, fair value	9,433	13,836
Continuous unrealized loss position less than 12 months, gross unrealized losses	267	592
Continuous unrealized loss position 12 months or longer, fair value	1,418	875
Continuous unrealized loss position 12 months or longer, gross unrealized losses	112	111
Continuous unrealized loss position, total, fair value	10,851	14,711
Continuous unrealized loss position, total, gross unrealized losses	379	703
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	9
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
Fixed maturities, amortized cost	61,995	62,196
Gross unrealized gains	2,651	2,460
Gross unrealized losses	375	700
Fixed maturities, at fair value	64,271	63,956
Continuous unrealized loss position less than 12 months, fair value	9,204	13,636
Continuous unrealized loss position less than 12 months, gross unrealized losses	263	589
Continuous unrealized loss position 12 months or longer, fair value	1,418	875
Continuous unrealized loss position 12 months or longer, gross unrealized losses	112	111
Continuous unrealized loss position, total, fair value	10,622	14,511
Continuous unrealized loss position, total, gross unrealized losses	375	700
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	9
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,192	2,288
Gross unrealized gains	40	39
Gross unrealized losses	9	12
Fixed maturities, at fair value	2,223	2,315
Continuous unrealized loss position less than 12 months, fair value	158	433
Continuous unrealized loss position less than 12 months, gross unrealized losses	9	12
Continuous unrealized loss position, total, fair value	158	433
Continuous unrealized loss position, total, gross unrealized losses	9	12
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	34,019	34,488
Gross unrealized gains	1,548	1,436
Gross unrealized losses	175	362
Fixed maturities, at fair value	35,392	35,562
Continuous unrealized loss position less than 12 months, fair value	3,143	4,785
Continuous unrealized loss position less than 12 months, gross unrealized losses	105	298
Continuous unrealized loss position 12 months or longer, fair value	890	432
Continuous unrealized loss position 12 months or longer, gross unrealized losses	70	64
Continuous unrealized loss position, total, fair value	4,033	5,217
Continuous unrealized loss position, total, gross unrealized losses	175	362
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	8,320	9,074
Gross unrealized gains	380	445
Gross unrealized losses	1	1
Fixed maturities, at fair value	8,699	9,518
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	25,699	25,414
Gross unrealized gains	1,168	991
Gross unrealized losses	174	361
Fixed maturities, at fair value	26,693	26,044
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,539	2,552
Gross unrealized gains	35	33
Gross unrealized losses	3	8
Fixed maturities, at fair value	2,571	2,577
Continuous unrealized loss position less than 12 months, fair value	520	907
Continuous unrealized loss position less than 12 months, gross unrealized losses	3	8
Continuous unrealized loss position 12 months or longer, fair value	4	1
Continuous unrealized loss position, total, fair value	524	908
Continuous unrealized loss position, total, gross unrealized losses	3	8
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,202	2,263
Gross unrealized gains	174	179
Gross unrealized losses	15	18
Fixed maturities, at fair value	2,361	2,424
Continuous unrealized loss position less than 12 months, fair value	508	542
Continuous unrealized loss position less than 12 months, gross unrealized losses	14	17
Continuous unrealized loss position 12 months or longer, fair value	20	21
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	1
Continuous unrealized loss position, total, fair value	528	563
Continuous unrealized loss position, total, gross unrealized losses	15	18
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	20,914	20,472
Gross unrealized gains	846	767
Gross unrealized losses	173	299
Fixed maturities, at fair value	21,587	20,940
Continuous unrealized loss position less than 12 months, fair value	4,875	6,887
Continuous unrealized loss position less than 12 months, gross unrealized losses	132	253
Continuous unrealized loss position 12 months or longer, fair value	504	421
Continuous unrealized loss position 12 months or longer, gross unrealized losses	41	46
Continuous unrealized loss position, total, fair value	5,379	7,308
Continuous unrealized loss position, total, gross unrealized losses	173	299
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	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	9
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	129	133
Gross unrealized gains	8	6
Gross unrealized losses		1
Fixed maturities, at fair value	137	138
Continuous unrealized loss position less than 12 months, fair value		82
Continuous unrealized loss position less than 12 months, gross unrealized losses		1
Continuous unrealized loss position, total, fair value		82
Continuous unrealized loss position, total, gross unrealized losses		1
Equity securities [Member]
Investment disclosure details
Equity securities, cost	660	686
Gross unrealized gains	282	260
Gross unrealized losses	4	3
Equity securities, at fair value	938	943
Continuous unrealized loss position less than 12 months, fair value	229	200
Continuous unrealized loss position less than 12 months, gross unrealized losses	4	3
Continuous unrealized loss position, total, fair value	229	200
Continuous unrealized loss position, total, gross unrealized losses	4	3
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	385	385
Gross unrealized gains	248	226
Gross unrealized losses	2	1
Equity securities, at fair value	631	610
Continuous unrealized loss position less than 12 months, fair value	53	53
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	1
Continuous unrealized loss position, total, fair value	53	53
Continuous unrealized loss position, total, gross unrealized losses	2	1
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	275	301
Gross unrealized gains	34	34
Gross unrealized losses	2	2
Equity securities, at fair value	307	333
Continuous unrealized loss position less than 12 months, fair value	176	147
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	2
Continuous unrealized loss position, total, fair value	176	147
Continuous unrealized loss position, total, gross unrealized losses	$ 2	$ 2