Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair value by pricing source
Available-for-sale securities	$ 10,407	$ 9,557
Indemnity
Fair value by pricing source
Available-for-sale securities	588	576
Exchange
Fair value by pricing source
Available-for-sale securities	9,819	8,981
Fair value measurements on a recurring basis
Fair value by pricing source
Total	13,654	12,875
Fair value measurements on a recurring basis | Indemnity
Fair value by pricing source
Available-for-sale securities	588	576
Total	607	594
Fair value measurements on a recurring basis | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	563	526
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	557
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	6
Fair value measurements on a recurring basis | Indemnity | Fixed maturities | Priced via internal modeling
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis | Indemnity | Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	12	25
Fair value measurements on a recurring basis | 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 | 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	13	25
Fair value measurements on a recurring basis | Indemnity | Common stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	13
Fair value measurements on a recurring basis | Indemnity | Real estate
Fair value by pricing source
Other investments	19	18
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	9,819	8,981
Total	13,047	12,281
Fair value measurements on a recurring basis | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	8,902	8,162
Fair value measurements on a recurring basis | Exchange | Fixed maturities | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	8,763
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	132
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	720	621
Fair value measurements on a recurring basis | Exchange | Nonredeemable preferred stock | Priced via pricing services
Fair value by pricing source
Available-for-sale securities	697
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	22
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	197	198
Trading and available-for-sale securities	3,322
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	3,307
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	15
Fair value measurements on a recurring basis | Exchange | Real estate
Fair value by pricing source
Other investments	103	98
Fair value measurements on a recurring basis | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	103
Fair value measurements on a recurring basis | Level 1
Fair value by pricing source
Total	3,656	3,654
Fair value measurements on a recurring basis | Level 1 | Indemnity
Fair value by pricing source
Available-for-sale securities	15	27
Total	15	27
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	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	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 | Common stock
Fair value by pricing source
Available-for-sale securities	13	25
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	13
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	531	440
Total	3,641	3,627
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	334	242
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	334
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	197	198
Trading and available-for-sale securities	3,307
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	3,307
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	9,776	9,057
Fair value measurements on a recurring basis | Level 2 | Indemnity
Fair value by pricing source
Available-for-sale securities	569	547
Total	569	547
Fair value measurements on a recurring basis | Level 2 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	559	524
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	557
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	2
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	10	23
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	8
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	9,207	8,510
Total	9,207	8,510
Fair value measurements on a recurring basis | Level 2 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	8,829	8,131
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	8,763
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	66
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	378	379
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	363
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 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	222	164
Fair value measurements on a recurring basis | Level 3 | Indemnity
Fair value by pricing source
Available-for-sale securities	4	2
Total	23	20
Fair value measurements on a recurring basis | Level 3 | Indemnity | Fixed maturities
Fair value by pricing source
Available-for-sale securities	4	2
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	4
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	0
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	19	18
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	81	31
Total	199	144
Fair value measurements on a recurring basis | Level 3 | Exchange | Fixed maturities
Fair value by pricing source
Available-for-sale securities	73	31
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	66
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	8	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	7
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	15
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	15
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate
Fair value by pricing source
Other investments	103	98
Fair value measurements on a recurring basis | Level 3 | Exchange | Real estate | Priced via unobservable inputs
Fair value by pricing source
Other investments	103
Fair value measurements on a nonrecurring basis
Fair value by pricing source
Total	$ 0