Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair value by pricing source
Available-for-sale securities	$ 9,317	$ 9,159
Indemnity
Fair value by pricing source
Available-for-sale securities	547	507
Exchange
Fair value by pricing source
Available-for-sale securities	8,770	8,652
Fair value measurements on a recurring basis
Fair value by pricing source
Total	12,381	11,704
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	547	507
Total	566	526
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	499	452
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	497
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	1
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	19	19
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	8,770	8,652
Total	11,815	11,178
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,921	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	7,836
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	71
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	14
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	611	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	597
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	14
Fair value measurements on a recurring basis | 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 | Exchange | Common stock
Fair value by pricing source
Available-for-sale securities	238	314
Trading and available-for-sale securities	3,183
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	3,169
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	14
Fair value measurements on a recurring basis | Exchange | Real estate
Fair value by pricing source
Other investments	100	109
Fair value measurements on a recurring basis | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	100
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total	3,426	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	468	513
Total	3,399	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	230	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	230
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	238	314
Trading and available-for-sale securities	3,169
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	3,169
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	8,785	8,553
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	518	473
Total	518	473
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	497	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	497
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	0
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	8,267	8,080
Total	8,267	8,080
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,886	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	7,836
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	50
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	381	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	367
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	14
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	170	206
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	2	4
Total	21	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	19	19
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	35	59
Total	149	183
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	35	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	21
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	14
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	0	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	0
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	14
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	14
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate
Fair value by pricing source
Other investments	100	109
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	100
Fair value measurements on a nonrecurring basis
Fair value by pricing source
Total	$ 0