Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value by pricing source
Available-for-sale securities	$ 8,823	$ 8,429
Indemnity
Fair value by pricing source
Available-for-sale securities	576	573
Exchange
Fair value by pricing source
Available-for-sale securities	8,247	7,856
Fair value measurements on a recurring basis
Fair value by pricing source
Total available-for-sale securities and trading securities	11,492	10,764
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	576	573
Trading securities	30	27
Total available-for-sale securities and trading securities	606	600
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	547	548
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	543
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	3
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	29	25
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	27
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Indemnity | Common stock
Fair value by pricing source
Trading securities	30	27
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	30
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Exchange
Fair value by pricing source
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
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,623	7,292
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,497
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	92
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	34
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	624	564
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	585
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	39
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Exchange | Common stock
Fair value by pricing source
Trading securities	2,639	2,308
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	2,626
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	13
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total available-for-sale securities and trading securities	2,938	2,547
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	8	10
Trading securities	30	27
Total available-for-sale securities and trading securities	38	37
Fair value measurements on a recurring basis | Level 1 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	8	10
Fair value measurements on a recurring basis | Level 1 | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	8
Fair value measurements on a recurring basis | Level 1 | Indemnity | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock
Fair value by pricing source
Trading securities	30	27
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	30
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange
Fair value by pricing source
Available-for-sale securities	274	214
Trading securities	2,626	2,296
Total available-for-sale securities and trading securities	2,900	2,510
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	11	26
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	11
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	263	188
Fair value measurements on a recurring basis | Level 1 | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	263
Fair value measurements on a recurring basis | Level 1 | Exchange | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock
Fair value by pricing source
Trading securities	2,626	2,296
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	2,626
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2
Fair value by pricing source
Total available-for-sale securities and trading securities	8,442	8,146
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	564	559
Trading securities	0	0
Total available-for-sale securities and trading securities	564	559
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	543	544
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	543
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	21	15
Fair value measurements on a recurring basis | Level 2 | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	19
Fair value measurements on a recurring basis | Level 2 | Indemnity | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis | Level 2 | Indemnity | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock
Fair value by pricing source
Trading securities	0	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange
Fair value by pricing source
Available-for-sale securities	7,878	7,587
Trading securities	0	0
Total available-for-sale securities and trading securities	7,878	7,587
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,523	7,216
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,486
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	37
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	355	371
Fair value measurements on a recurring basis | Level 2 | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	322
Fair value measurements on a recurring basis | Level 2 | Exchange | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	33
Fair value measurements on a recurring basis | Level 2 | Exchange | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock
Fair value by pricing source
Trading securities	0	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 3
Fair value by pricing source
Total available-for-sale securities and trading securities	112	71
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	4	4
Trading securities	0	0
Total available-for-sale securities and trading securities	4	4
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	4	4
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	3
Fair value measurements on a recurring basis | Level 3 | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common stock
Fair value by pricing source
Trading securities	0	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange
Fair value by pricing source
Available-for-sale securities	95	55
Trading securities	13	12
Total available-for-sale securities and trading securities	108	67
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	89	50
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	55
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	34
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	6	5
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	6
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock
Fair value by pricing source
Trading securities	13	12
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading securities	13
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	$ 0