Fair Value (Details 4) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair value by pricing source
Available-for-sale securities	$ 10,772	$ 10,446
Indemnity
Fair value by pricing source
Available-for-sale securities	583	589
Exchange
Fair value by pricing source
Available-for-sale securities	10,189	9,857
Fair value measurements on a recurring basis
Fair value by pricing source
Total	13,975	13,748
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	583	589
Total	589	597
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	561	564
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	560
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 | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	10	12
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	8
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 | Common stock
Fair value by pricing source
Available-for-sale securities	12	13
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	12
Fair value measurements on a recurring basis | Indemnity | Real estate
Fair value by pricing source
Other investments	6	8
Fair value measurements on a recurring basis | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	6
Fair value measurements on a recurring basis | Exchange
Fair value by pricing source
Available-for-sale securities	10,189	9,857
Total	13,386	13,151
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	9,372	9,007
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	9,273
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	92
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	7
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	721	710
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	711
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	9
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis | Exchange | Common stock
Fair value by pricing source
Available-for-sale securities	96	140
Trading and available-for-sale securities	3,240
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	3,227
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	13
Fair value measurements on a recurring basis | Exchange | Real estate
Fair value by pricing source
Other investments	53	71
Fair value measurements on a recurring basis | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	53
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total	3,599	3,691
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	13	15
Total	13	15
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 | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	1	2
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	1
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 | Common stock
Fair value by pricing source
Available-for-sale securities	12	13
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	12
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	455	468
Total	3,586	3,676
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	359	328
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	359
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	96	140
Trading and available-for-sale securities	3,227
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	3,227
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	10,217	9,863
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	569	574
Total	569	574
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	560	564
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	560
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 | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	9	10
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	7
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 | 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 | 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	9,648	9,289
Total	9,648	9,289
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	9,287	8,908
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	9,262
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	25
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	361	381
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	352
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	9
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 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	159	194
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	1	0
Total	7	8
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	1	0
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 | 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 | 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 | Real estate
Fair value by pricing source
Other investments	6	8
Fair value measurements on a recurring basis | Level 3 | Indemnity | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	6
Fair value measurements on a recurring basis | Level 3 | Exchange
Fair value by pricing source
Available-for-sale securities	86	100
Total	152	186
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	85	99
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	11
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	67
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	7
Fair value measurements on a recurring basis | Level 3 | Exchange | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	1	1
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	1
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	13
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 internal modeling
Fair value by pricing source
Trading and available-for-sale securities	13
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate
Fair value by pricing source
Other investments	53	$ 71
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	53
Fair value measurements on a nonrecurring basis
Fair value by pricing source
Total	$ 0