Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair value by pricing source
Available-for-sale securities	$ 8,710	$ 8,429
Indemnity
Fair value by pricing source
Available-for-sale securities	536	573
Exchange
Fair value by pricing source
Available-for-sale securities	8,174	7,856
Fair value measurements on a recurring basis | Total
Fair value by pricing source
Total available-for-sale securities and trading securities	11,228	10,764
Fair value measurements on a recurring basis | Total | Indemnity
Fair value by pricing source
Available-for-sale securities	536	573
Trading securities	28	27
Total available-for-sale securities and trading securities	564	600
Fair value measurements on a recurring basis | Total | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	510	548
Fair value measurements on a recurring basis | Total | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	505
Fair value measurements on a recurring basis | Total | 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 | Total | Indemnity | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	4
Fair value measurements on a recurring basis | Total | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	26	25
Fair value measurements on a recurring basis | Total | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	24
Fair value measurements on a recurring basis | Total | 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 | Total | 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 | Total | Indemnity | Common stock
Fair value by pricing source
Trading securities	28	27
Fair value measurements on a recurring basis | Total | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	28
Fair value measurements on a recurring basis | Total | 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 | Total | Indemnity | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	0
Fair value measurements on a recurring basis | Total | Exchange
Fair value by pricing source
Available-for-sale securities	8,174	7,856
Trading securities	2,490	2,308
Total available-for-sale securities and trading securities	10,664	10,164
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,569	7,292
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,423
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	110
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	36
Fair value measurements on a recurring basis | Total | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	605	564
Fair value measurements on a recurring basis | Total | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	587
Fair value measurements on a recurring basis | Total | Exchange | Nonredeemable preferred stock | Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	18
Fair value measurements on a recurring basis | Total | 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 | Total | Exchange | Common stock
Fair value by pricing source
Trading securities	2,490	2,308
Fair value measurements on a recurring basis | Total | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	2,476
Fair value measurements on a recurring basis | Total | 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 | Total | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	14
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total available-for-sale securities and trading securities	2,779	2,547
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	10	10
Trading securities	28	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	10	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	10
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	28	27
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	28
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	265	214
Trading securities	2,476	2,296
Total available-for-sale securities and trading securities	2,741	2,510
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	26	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	26
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	239	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	239
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	2,476	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	2,476
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	8,366	8,146
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	521	559
Trading securities	0	0
Total available-for-sale securities and trading securities	521	559
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	505	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	505
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	16	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	14
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	7,845	7,587
Trading securities	0	0
Total available-for-sale securities and trading securities	7,845	7,587
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,484	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	7,397
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	87
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	361	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	348
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	13
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	83	71
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	5	4
Trading securities	0	0
Total available-for-sale securities and trading securities	5	4
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	5	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	4
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	64	55
Trading securities	14	12
Total available-for-sale securities and trading securities	78	67
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	23
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	36
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	5	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	5
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	14	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	0
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock | Priced via unobservable inputs
Fair value by pricing source
Trading securities	$ 14