Fair Value (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Level 3
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	$ 238	$ 186	$ 190	$ 171
Included in earnings	4	5	16	16
Included in other comprehensive income	1	(1)	3	(3)
Purchases	1	1	2	13
Sales	(15)	(1)	(28)	(7)
Transfers in and (out) of Level 3	(23)	0	23	0
Ending balance	206	190	206	190
Level 3 | Net realized investment gains (losses)
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Included in earnings	1	(1)	3	0
Level 3 | Net investment income
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Included in earnings		0		0
Level 3 | Equity in earnings of limited partnerships
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Included in earnings	3	6	13	16
Indemnity
Fair value transfers due to changes in available market observable inputs
Transfers from level 1 to level 2	4		5
Indemnity | Nonredeemable preferred stock
Fair value transfers due to changes in available market observable inputs
No. of securities transferred between Level 1 and Level 2	2		2
Indemnity | Level 3
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	22	21	21	20
Included in earnings	1	0	2	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	1
Sales	0	0	(1)	0
Transfers in and (out) of Level 3	0	0	1	0
Ending balance	23	21	23	21
Indemnity | Level 3 | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	4	4	4	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(1)	0
Transfers in and (out) of Level 3	0	0	1	0
Ending balance	4	4	4	4
Indemnity | Level 3 | Fixed maturities | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	4	4	4	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(1)	0
Transfers in and (out) of Level 3	0	0	1	0
Ending balance	4	4	4	4
Indemnity | Level 3 | Corporate debt securities | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	1		0
Included in earnings	0		0
Included in other comprehensive income	0		0
Purchases	0		0
Sales	0		0
Transfers in and (out) of Level 3	0		1
Ending balance	1		1
Indemnity | Level 3 | Collateralized debt obligations (CDO) | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	3	4	4	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(1)	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	3	4	3	4
Indemnity | Level 3 | Real estate | Limited partnerships
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	18	17	17	16
Included in earnings	1	0	2	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	1
Sales	0	0	0	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	19	17	19	17
Exchange
Fair value transfers due to changes in available market observable inputs
Transfers from level 1 to level 2	69		79
Transfers from level 2 to level 1			12
Exchange | U.S. government & agencies
Fair value transfers due to changes in available market observable inputs
No. of securities transferred between Level 1 and Level 2	7		8
Exchange | Nonredeemable preferred stock
Fair value transfers due to changes in available market observable inputs
No. of securities transferred between Level 1 and Level 2	10		8
No. of securities transferred between Level 2 and Level 1			1
Exchange | Level 3
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	216	165	169	151
Included in earnings	3	5	14	16
Included in other comprehensive income	1	(1)	3	(3)
Purchases	1	1	2	12
Sales	(15)	(1)	(27)	(7)
Transfers in and (out) of Level 3	(23)	0	22	0
Ending balance	183	169	183	169
Exchange | Level 3 | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	95	56	55	62
Included in earnings	(1)	0	0	0
Included in other comprehensive income	1	(1)	3	(3)
Purchases	0	0	1	1
Sales	(13)	0	(22)	(5)
Transfers in and (out) of Level 3	(23)	0	22	0
Ending balance	59	55	59	55
Exchange | Level 3 | Trading securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	13	13	12	12
Included in earnings	2	(1)	3	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	15	12	15	12
Exchange | Level 3 | Fixed maturities | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	89	50	50	55
Included in earnings	(1)	0	0	0
Included in other comprehensive income	1	0	3	(1)
Purchases	0	0	1	1
Sales	(13)	0	(22)	(5)
Transfers in and (out) of Level 3	(17)	0	27	0
Ending balance	59	50	59	50
Exchange | Level 3 | States & political subdivisions | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	4	4	4	4
Included in earnings	(1)	0	(1)	0
Included in other comprehensive income	1	0	1	0
Purchases	0	0	0	0
Sales	(4)	0	(4)	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	0	4	0	4
Exchange | Level 3 | Corporate debt securities | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	34	12	12	11
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	2	0
Purchases	0	0	1	1
Sales	0	0	(5)	0
Transfers in and (out) of Level 3	9	0	33	0
Ending balance	43	12	43	12
Exchange | Level 3 | Collateralized debt obligations (CDO) | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	46	29	29	30
Included in earnings	0	0	1	0
Included in other comprehensive income	0	0	0	(1)
Purchases	0	0	0	0
Sales	(9)	0	(13)	0
Transfers in and (out) of Level 3	(21)	0	(1)	0
Ending balance	16	29	16	29
Exchange | Level 3 | Other debt securities | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	5	5	5	10
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	(5)
Transfers in and (out) of Level 3	(5)	0	(5)	0
Ending balance	0	5	0	5
Exchange | Level 3 | Nonredeemable preferred stock | Available-for-sale securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	6	6	5	7
Included in earnings	0	0	0	0
Included in other comprehensive income	0	(1)	0	(2)
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers in and (out) of Level 3	(6)	0	(5)	0
Ending balance	0	5	0	5
Exchange | Level 3 | Common stock | Trading securities
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	13	13	12	12
Included in earnings	2	(1)	3	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	15	12	15	12
Exchange | Level 3 | Real estate | Limited partnerships
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Beginning balance	108	96	102	77
Included in earnings	2	6	11	16
Included in other comprehensive income	0	0	0	0
Purchases	1	1	1	11
Sales	(2)	(1)	(5)	(2)
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	109	102	109	102
Fair value measurements on a recurring basis
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Ending balance	78		78
Fair value measurements on a recurring basis | Real estate | Limited partnerships
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Ending balance	128		128
Fair value measurements on a recurring basis | Indemnity
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Ending balance	4		4
Fair value measurements on a recurring basis | Exchange
Roll forward of Level 3 consolidated fair value measurements on a recurring basis
Ending balance	$ 74		$ 74