Fair Value (Details 2) (Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	$ 73	$ 83	$ 78	$ 95
Transfer in and (out)		0		0
Included in earnings	(1)	0	0	2
Included in other comprehensive income	(1)	2	(3)	7
Purchases, sales and adjustments		(17)		(19)
Purchases	0		1
Sales	0		(5)
Transfers in and (out) of Level 3	0	10	0	(7)
Ending balance	71	78	71	78
Price difference between the pricing service and the observable market data			0.1
Indemnity
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	4	11	4	11
Transfer in and (out)		(7)		(7)
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	1	0	0
Ending balance	4	4	4	4
Indemnity | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	4	11	4	11
Transfer in and (out)		(7)		(7)
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		(1)		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	1	0	0
Ending balance	4	4	4	4
Indemnity | Trading securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	0	0	0	0
Transfer in and (out)		0		0
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	0	0	0	0
Indemnity | Fixed maturities | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	4	9	4	10
Transfer in and (out)		(6)		(6)
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	1	0	0
Ending balance	4	4	4	4
Indemnity | Corporate debt securities | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance		2		2
Transfer in and (out)		(2)		(2)
Included in earnings		0		0
Included in other comprehensive income		0		0
Purchases, sales and adjustments		0		0
Transfers in and (out) of Level 3		0		0
Ending balance		0		0
Indemnity | Collateralized debt obligations (CDO) | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	4	7	4	8
Transfer in and (out)		(4)		(4)
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	1	0	0
Ending balance	4	4	4	4
Indemnity | Nonredeemable preferred stock | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance		2		1
Transfer in and (out)		(1)		(1)
Included in earnings		0		0
Included in other comprehensive income		0		0
Purchases, sales and adjustments		(1)		0
Transfers in and (out) of Level 3		0		0
Ending balance		0		0
Indemnity | Common stock | Trading securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	0	0	0	0
Transfer in and (out)	0	0	0	0
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	0	0	0	0
Exchange
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	69	72	74	84
Transfer in and (out)		7		7
Included in earnings	(1)	0	0	2
Included in other comprehensive income	(1)	2	(3)	7
Purchases, sales and adjustments		(16)		(19)
Purchases	0		1
Sales	0		(5)
Transfers in and (out) of Level 3	0	9	0	(7)
Ending balance	67	74	67	74
Exchange | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	56	61	62	75
Transfer in and (out)		7		7
Included in earnings	0	(1)	0	(1)
Included in other comprehensive income	(1)	2	(3)	7
Purchases, sales and adjustments	0	(16)		(19)
Purchases	0		1
Sales	0		(5)
Transfers in and (out) of Level 3	0	9	0	(7)
Ending balance	55	62	55	62
Exchange | Trading securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	13	11	12	9
Transfer in and (out)		0		0
Included in earnings	(1)	1	0	3
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	12	12	12	12
Exchange | Fixed maturities | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	50	56	55	71
Transfer in and (out)		6		6
Included in earnings	0	(1)	0	(1)
Included in other comprehensive income	0	1	(1)	5
Purchases, sales and adjustments		(16)		(19)
Purchases	0		1
Sales	0		(5)
Transfers in and (out) of Level 3	0	9	0	(7)
Ending balance	50	55	50	55
Exchange | States & political subdivisions | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	4	0	4	0
Transfer in and (out)		0		0
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	4	0	4
Ending balance	4	4	4	4
Exchange | Corporate debt securities | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	12	9	11	17
Transfer in and (out)		2		2
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		1
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	(8)
Ending balance	12	11	12	11
Exchange | Other debt securities | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	5	5	10	5
Transfer in and (out)		0		0
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		(5)
Transfers in and (out) of Level 3	0	5	0	5
Ending balance	5	10	5	10
Exchange | Collateralized debt obligations (CDO) | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	29	42	30	49
Transfer in and (out)		4		4
Included in earnings	0	(1)	0	(1)
Included in other comprehensive income	0	1	(1)	5
Purchases, sales and adjustments		(16)		(19)
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	(8)
Ending balance	29	30	29	30
Exchange | Nonredeemable preferred stock | Available-for-sale securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	6	5	7	4
Transfer in and (out)		1		1
Included in earnings	0	0	0	0
Included in other comprehensive income	(1)	1	(2)	2
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	5	7	5	7
Exchange | Common stock | Trading securities
Roll forward of Level 3 fair value measurements on a recurring basis for consolidated available-for-sale and trading securities
Beginning balance	13	11	12	9
Transfer in and (out)		0		0
Included in earnings	(1)	1	0	3
Included in other comprehensive income	0	0	0	0
Purchases, sales and adjustments		0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	$ 12	$ 12	$ 12	$ 12