Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair value by pricing source
Available-for-sale securities	$ 8,624	$ 8,429
Indemnity
Fair value by pricing source
Available-for-sale securities	520	573
Exchange
Fair value by pricing source
Available-for-sale securities	8,104	7,856
Fair value measurements on a recurring basis | Total
Fair value by pricing source
Total available-for-sale securities and trading securities	11,249	10,764
Fair value measurements on a recurring basis | Total | Indemnity
Fair value by pricing source
Available-for-sale securities	520	573
Trading securities	29	27
Total available-for-sale securities and trading securities	549	600
Fair value measurements on a recurring basis | Total | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	494	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	489
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	29	27
Fair value measurements on a recurring basis | Total | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	29
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	8,104	7,856
Trading securities	2,596	2,308
Total available-for-sale securities and trading securities	10,700	10,164
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,483	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	7,324
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	114
Fair value measurements on a recurring basis | Total | Exchange | Fixed maturities | Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	45
Fair value measurements on a recurring basis | Total | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	621	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	600
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	15
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	6
Fair value measurements on a recurring basis | Total | Exchange | Common stock
Fair value by pricing source
Trading securities	2,596	2,308
Fair value measurements on a recurring basis | Total | Exchange | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	2,582
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	2,874	2,547
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	11	10
Trading securities	29	27
Total available-for-sale securities and trading securities	40	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	11	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	11
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	29	27
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Trading securities	29
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	252	214
Trading securities	2,582	2,296
Total available-for-sale securities and trading securities	2,834	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	226	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	226
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	2,582	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	2,582
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	8,287	8,146
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	504	559
Trading securities	0	0
Total available-for-sale securities and trading securities	504	559
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	489	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	489
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	15	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	13
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	7,783	7,587
Trading securities	0	0
Total available-for-sale securities and trading securities	7,783	7,587
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,394	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	7,298
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	96
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	389	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	374
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	15
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	88	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
Total available-for-sale securities and trading securities		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	69	55
Trading securities	14	12
Total available-for-sale securities and trading securities	83	67
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	63	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	18
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	45
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	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	6
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