Investments (details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment disclosure details
Fixed maturities, amortized cost	$ 61,043	$ 62,196
Fixed maturities, at fair value	63,622	63,956
Equity securities, cost	609	686
Equity securities, at fair value	949	943
Continuous unrealized loss position less than 12 months, fair value	4,893	13,836
Continuous unrealized loss position less than 12 months, gross unrealized losses	40	592
Continuous unrealized loss position 12 months or longer, fair value	4,900	875
Continuous unrealized loss position 12 months or longer, gross unrealized losses	136	111
Continuous unrealized loss position, total, fair value	9,793	14,711
Continuous unrealized loss position, total, gross unrealized losses	176	703
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
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	61,043	62,196
Gross unrealized gains	2,753	2,460
Gross unrealized losses	174	700
Fixed maturities, at fair value	63,622	63,956
Continuous unrealized loss position less than 12 months, fair value	4,749	13,636
Continuous unrealized loss position less than 12 months, gross unrealized losses	38	589
Continuous unrealized loss position 12 months or longer, fair value	4,899	875
Continuous unrealized loss position 12 months or longer, gross unrealized losses	136	111
Continuous unrealized loss position, total, fair value	9,648	14,511
Continuous unrealized loss position, total, gross unrealized losses	174	700
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,050	2,288
Gross unrealized gains	31	39
Gross unrealized losses	7	12
Fixed maturities, at fair value	2,074	2,315
Continuous unrealized loss position less than 12 months, fair value	172	433
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	12
Continuous unrealized loss position 12 months or longer, fair value	105
Continuous unrealized loss position 12 months or longer, gross unrealized losses	5
Continuous unrealized loss position, total, fair value	277	433
Continuous unrealized loss position, total, gross unrealized losses	7	12
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	32,246	34,488
Gross unrealized gains	1,684	1,436
Gross unrealized losses	33	362
Fixed maturities, at fair value	33,897	35,562
Continuous unrealized loss position less than 12 months, fair value	1,033	4,785
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	298
Continuous unrealized loss position 12 months or longer, fair value	1,736	432
Continuous unrealized loss position 12 months or longer, gross unrealized losses	32	64
Continuous unrealized loss position, total, fair value	2,769	5,217
Continuous unrealized loss position, total, gross unrealized losses	33	362
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	7,459	9,074
Gross unrealized gains	348	445
Gross unrealized losses		1
Fixed maturities, at fair value	7,807	9,518
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	24,787	25,414
Gross unrealized gains	1,336	991
Gross unrealized losses	33	361
Fixed maturities, at fair value	26,090	26,044
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,435	2,552
Gross unrealized gains	34	33
Gross unrealized losses	1	8
Fixed maturities, at fair value	2,468	2,577
Continuous unrealized loss position less than 12 months, fair value	226	907
Continuous unrealized loss position less than 12 months, gross unrealized losses		8
Continuous unrealized loss position 12 months or longer, fair value	143	1
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	369	908
Continuous unrealized loss position, total, gross unrealized losses	1	8
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,114	2,263
Gross unrealized gains	167	179
Gross unrealized losses	7	18
Fixed maturities, at fair value	2,274	2,424
Continuous unrealized loss position less than 12 months, fair value	157	542
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	17
Continuous unrealized loss position 12 months or longer, fair value	257	21
Continuous unrealized loss position 12 months or longer, gross unrealized losses	6	1
Continuous unrealized loss position, total, fair value	414	563
Continuous unrealized loss position, total, gross unrealized losses	7	18
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	22,072	20,472
Gross unrealized gains	828	767
Gross unrealized losses	126	299
Fixed maturities, at fair value	22,774	20,940
Continuous unrealized loss position less than 12 months, fair value	3,161	6,887
Continuous unrealized loss position less than 12 months, gross unrealized losses	34	253
Continuous unrealized loss position 12 months or longer, fair value	2,658	421
Continuous unrealized loss position 12 months or longer, gross unrealized losses	92	46
Continuous unrealized loss position, total, fair value	5,819	7,308
Continuous unrealized loss position, total, gross unrealized losses	126	299
Other fixed maturities [Member]
Investment disclosure details
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	126	133
Gross unrealized gains	9	6
Gross unrealized losses		1
Fixed maturities, at fair value	135	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	609	686
Gross unrealized gains	342	260
Gross unrealized losses	2	3
Equity securities, at fair value	949	943
Continuous unrealized loss position less than 12 months, fair value	144	200
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	3
Continuous unrealized loss position 12 months or longer, fair value	1
Continuous unrealized loss position, total, fair value	145	200
Continuous unrealized loss position, total, gross unrealized losses	2	3
Equity securities, public common stock [Member]
Investment disclosure details
Equity securities, cost	396	385
Gross unrealized gains	312	226
Gross unrealized losses		1
Equity securities, at fair value	708	610
Continuous unrealized loss position less than 12 months, fair value	36	53
Continuous unrealized loss position less than 12 months, gross unrealized losses		1
Continuous unrealized loss position 12 months or longer, fair value	1
Continuous unrealized loss position, total, fair value	37	53
Continuous unrealized loss position, total, gross unrealized losses		1
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	213	301
Gross unrealized gains	30	34
Gross unrealized losses	2	2
Equity securities, at fair value	241	333
Continuous unrealized loss position less than 12 months, fair value	108	147
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	2
Continuous unrealized loss position, total, fair value	108	147
Continuous unrealized loss position, total, gross unrealized losses	$ 2	$ 2