Fair Value (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	$ 8,823	$ 8,429
Fair value measurements on a recurring basis
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Total available-for-sale securities and trading securities	11,492	10,764
Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Total available-for-sale securities and trading securities	2,938	2,547
Fair value measurements on a recurring basis | Level 2
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Total available-for-sale securities and trading securities	8,442	8,146
Fair value measurements on a recurring basis | Level 3
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Total available-for-sale securities and trading securities	112	71
Indemnity
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	576	573
Indemnity | Fair value measurements on a recurring basis
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	576	573
Trading securities	30	27
Total available-for-sale securities and trading securities	606	600
Indemnity | Fair value measurements on a recurring basis | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	547	548
Indemnity | Fair value measurements on a recurring basis | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	195	221
Indemnity | Fair value measurements on a recurring basis | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	335	303
Indemnity | Fair value measurements on a recurring basis | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	6	13
Indemnity | Fair value measurements on a recurring basis | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	3	4
Indemnity | Fair value measurements on a recurring basis | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8	7
Indemnity | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	29	25
Indemnity | Fair value measurements on a recurring basis | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	30	27
Indemnity | Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8	10
Trading securities	30	27
Total available-for-sale securities and trading securities	38	37
Indemnity | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8	10
Indemnity | Fair value measurements on a recurring basis | Level 1 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	30	27
Indemnity | Fair value measurements on a recurring basis | Level 2
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	564	559
Trading securities	0	0
Total available-for-sale securities and trading securities	564	559
Indemnity | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	543	544
Indemnity | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	195	221
Indemnity | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	334	303
Indemnity | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	6	13
Indemnity | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8	7
Indemnity | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	21	15
Indemnity | Fair value measurements on a recurring basis | Level 2 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	4	4
Trading securities	0	0
Total available-for-sale securities and trading securities	4	4
Indemnity | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	4	4
Indemnity | Fair value measurements on a recurring basis | Level 3 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	1	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	3	4
Indemnity | Fair value measurements on a recurring basis | Level 3 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	0	0
Exchange
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8,247	7,856
Exchange | Fair value measurements on a recurring basis
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	8,247	7,856
Trading securities	2,639	2,308
Total available-for-sale securities and trading securities	10,886	10,164
Exchange | Fair value measurements on a recurring basis | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	7,623	7,292
Exchange | Fair value measurements on a recurring basis | U.S. government & agencies
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	17	17
Exchange | Fair value measurements on a recurring basis | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	1,348	1,379
Exchange | Fair value measurements on a recurring basis | Foreign government securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	16	15
Exchange | Fair value measurements on a recurring basis | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	5,886	5,499
Exchange | Fair value measurements on a recurring basis | Residential mortgage-backed securities (RMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	174	189
Exchange | Fair value measurements on a recurring basis | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	72	66
Exchange | Fair value measurements on a recurring basis | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	62	65
Exchange | Fair value measurements on a recurring basis | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	48	62
Exchange | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	624	564
Exchange | Fair value measurements on a recurring basis | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	2,639	2,308
Exchange | Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	274	214
Trading securities	2,626	2,296
Total available-for-sale securities and trading securities	2,900	2,510
Exchange | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	11	26
Exchange | Fair value measurements on a recurring basis | Level 1 | U.S. government & agencies
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	5	6
Exchange | Fair value measurements on a recurring basis | Level 1 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Foreign government securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	6	20
Exchange | Fair value measurements on a recurring basis | Level 1 | Residential mortgage-backed securities (RMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	263	188
Exchange | Fair value measurements on a recurring basis | Level 1 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	2,626	2,296
Exchange | Fair value measurements on a recurring basis | Level 2
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	7,878	7,587
Trading securities	0	0
Total available-for-sale securities and trading securities	7,878	7,587
Exchange | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	7,523	7,216
Exchange | Fair value measurements on a recurring basis | Level 2 | U.S. government & agencies
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	12	11
Exchange | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	1,344	1,375
Exchange | Fair value measurements on a recurring basis | Level 2 | Foreign government securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	16	15
Exchange | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	5,846	5,467
Exchange | Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities (RMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	174	189
Exchange | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	72	66
Exchange | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	16	36
Exchange | Fair value measurements on a recurring basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	43	57
Exchange | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	355	371
Exchange | Fair value measurements on a recurring basis | Level 2 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	95	55
Trading securities	13	12
Total available-for-sale securities and trading securities	108	67
Exchange | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	89	50
Exchange | Fair value measurements on a recurring basis | Level 3 | U.S. government & agencies
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | States & political subdivisions
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	4	4
Exchange | Fair value measurements on a recurring basis | Level 3 | Foreign government securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Corporate debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	34	12
Exchange | Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities (RMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities (CMBS)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations (CDO)
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	46	29
Exchange | Fair value measurements on a recurring basis | Level 3 | Other debt securities
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	5	5
Exchange | Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Available-for-sale securities	6	5
Exchange | Fair value measurements on a recurring basis | Level 3 | Common stock
Fair value measurements on a recurring basis for consolidated available-for-sale and trading securities by asset class and level of input
Trading securities	$ 13	$ 12