Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair value by pricing source
Available-for-sale securities	$ 9,180	$ 9,159
Indemnity
Fair value by pricing source
Available-for-sale securities	486	507
Exchange
Fair value by pricing source
Available-for-sale securities	8,694	8,652
Fair value measurements on a recurring basis
Fair value by pricing source
Total	12,037	11,704
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	486	507
Total	506	526
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	438	452
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	426
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	11
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	23	29
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	21
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via internal modeling
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	25	26
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	25
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via internal modeling
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	20	19
Fair value measurements on a recurring basis | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	20
Fair value measurements on a recurring basis | Exchange
Fair value by pricing source
Available-for-sale securities	8,694	8,652
Total	11,531	11,178
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,725	7,707
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,580
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	128
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	17
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	647	631
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	625
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	15
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	7
Fair value measurements on a recurring basis | Exchange | Common stock
Fair value by pricing source
Available-for-sale securities	322	314
Trading and available-for-sale securities	3,046
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	3,039
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via market comparables/broker quotes
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis | Exchange | Common stock | Priced via internal modeling
Fair value by pricing source
Trading and available-for-sale securities	7
Fair value measurements on a recurring basis | Exchange | Real estate
Fair value by pricing source
Other investments	113	109
Fair value measurements on a recurring basis | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	113
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total	3,297	2,945
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	27	30
Total	27	30
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/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Fixed maturities | Priced via internal modeling
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	2	4
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	2
Fair value measurements on a recurring basis | Level 1 | Indemnity | Nonredeemable preferred stock | Priced via market comparables/broker quotes
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 internal modeling
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	25	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	25
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Indemnity | Common stock | Priced via internal modeling
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	553	513
Total	3,270	2,915
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	0	0
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/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Exchange | Fixed maturities | Priced via internal modeling
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	231	199
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	231
Fair value measurements on a recurring basis | Level 1 | Exchange | Nonredeemable preferred stock | Priced via market comparables/broker quotes
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 internal modeling
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	322	314
Trading and available-for-sale securities	3,039
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	3,039
Fair value measurements on a recurring basis | Level 1 | Exchange | Common stock | Priced via market comparables/broker quotes
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 internal modeling
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	8,542	8,553
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	457	473
Total	457	473
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	436	448
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	426
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	10
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities | Priced via internal modeling
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	25
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/broker quotes
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 internal modeling
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	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/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 2 | Indemnity | Common stock | Priced via internal modeling
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	8,085	8,080
Total	8,085	8,080
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,676	7,648
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	7,580
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities | Priced via market comparables/broker quotes
Fair value by pricing source
Available-for-sale securities	96
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities | Priced via internal modeling
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	409	432
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	394
Fair value measurements on a recurring basis | Level 2 | Exchange | Nonredeemable preferred stock | Priced via market comparables/broker quotes
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 internal modeling
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	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/broker quotes
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 internal modeling
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	198	206
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	2	4
Total	22	23
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	2	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/broker quotes
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	1
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/broker quotes
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 internal modeling
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	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/broker quotes
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Indemnity | Common stock | Priced via internal modeling
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	20	19
Fair value measurements on a recurring basis | Level 3 | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	20
Fair value measurements on a recurring basis | Level 3 | Exchange
Fair value by pricing source
Available-for-sale securities	56	59
Total	176	183
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	49	59
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/broker quotes
Fair value by pricing source
Available-for-sale securities	32
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	17
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	7	0
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/broker quotes
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 internal modeling
Fair value by pricing source
Available-for-sale securities	7
Fair value measurements on a recurring basis | Level 3 | Exchange | Common stock
Fair value by pricing source
Available-for-sale securities	0	0
Trading and available-for-sale securities	7
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/broker quotes
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 internal modeling
Fair value by pricing source
Trading and available-for-sale securities	7
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate
Fair value by pricing source
Other investments	113	109
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	113
Fair value measurements on a nonrecurring basis
Fair value by pricing source
Total	$ 0