Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair value by pricing source
Available-for-sale securities	$ 9,159	$ 8,429
Indemnity
Fair value by pricing source
Available-for-sale securities	507	573
Exchange
Fair value by pricing source
Available-for-sale securities	8,652	7,856
Fair value measurements on a recurring basis
Fair value by pricing source
Total	11,704	10,883
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	507	573
Total	526	617
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	452	548
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	434
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	15
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
Available-for-sale securities	26
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	26
Fair value measurements on a recurring basis | Indemnity | Common 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 | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Indemnity | Real estate
Fair value by pricing source
Other investments	19	17
Fair value measurements on a recurring basis | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	19
Fair value measurements on a recurring basis | Exchange
Fair value by pricing source
Available-for-sale securities	8,652	7,856
Total	11,178	10,266
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,707	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	7,517
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	165
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	25
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	631	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	591
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	40
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
Available-for-sale securities	314
Trading and available-for-sale securities	2,731
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	2,716
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	15
Fair value measurements on a recurring basis | Exchange | Real estate
Fair value by pricing source
Other investments	109	102
Fair value measurements on a recurring basis | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	109
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total	2,945	2,547
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	30	10
Total	30	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	4	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	4
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
Available-for-sale securities	26
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	26
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
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common 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 | Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis | Level 1 | Exchange
Fair value by pricing source
Available-for-sale securities	513	214
Total	2,915	2,510
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	0	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	0
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	199	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	199
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
Available-for-sale securities	314
Trading and available-for-sale securities	2,716
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	2,716
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 and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis | Level 2
Fair value by pricing source
Total	8,553	8,146
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	473	559
Total	473	559
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	448	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	434
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	14
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	25	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	23
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
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale 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
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common 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 | Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis | Level 2 | Exchange
Fair value by pricing source
Available-for-sale securities	8,080	7,587
Total	8,080	7,587
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,648	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	7,517
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	131
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	432	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	392
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	40
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
Available-for-sale securities	0
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading and available-for-sale 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 and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis | Level 2 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis | Level 3
Fair value by pricing source
Total	206	190
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	4	4
Total	23	21
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
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common 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 | Common 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 | Common 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 | Real estate
Fair value by pricing source
Other investments	19	17
Fair value measurements on a recurring basis | Level 3 | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	19
Fair value measurements on a recurring basis | Level 3 | Exchange
Fair value by pricing source
Available-for-sale securities	59	55
Total	183	169
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	59	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	34
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	25
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	0	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	0
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
Available-for-sale securities	0
Trading and available-for-sale securities	15
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading and available-for-sale 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 and available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	15
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate
Fair value by pricing source
Other investments	109	102
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	$ 109