Fair Value (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Level 3	Mar. 31, 2012 Level 3	Mar. 31, 2013 Level 3 Net realized investment gains losses	Mar. 31, 2012 Level 3 Net realized investment gains losses	Mar. 31, 2013 Level 3 Equity in earnings of limited partnerships	Mar. 31, 2012 Level 3 Equity in earnings of limited partnerships	Mar. 31, 2013 Indemnity	Mar. 31, 2012 Indemnity	Mar. 31, 2012 Indemnity Fixed maturities Holdings	Mar. 31, 2013 Indemnity Nonredeemable preferred stock Holdings	Mar. 31, 2013 Indemnity Level 3	Mar. 31, 2012 Indemnity Level 3	Mar. 31, 2013 Indemnity Level 3 Available-for-sale securities	Mar. 31, 2012 Indemnity Level 3 Available-for-sale securities	Mar. 31, 2013 Indemnity Level 3 Fixed maturities Available-for-sale securities	Mar. 31, 2012 Indemnity Level 3 Fixed maturities Available-for-sale securities	Mar. 31, 2013 Indemnity Level 3 Corporate debt securities Available-for-sale securities	Mar. 31, 2012 Indemnity Level 3 Corporate debt securities Available-for-sale securities	Mar. 31, 2013 Indemnity Level 3 Collateralized debt obligations Available-for-sale securities	Mar. 31, 2012 Indemnity Level 3 Collateralized debt obligations Available-for-sale securities	Mar. 31, 2013 Indemnity Level 3 Real estate Limited partnerships	Mar. 31, 2012 Indemnity Level 3 Real estate Limited partnerships	Mar. 31, 2013 Exchange	Mar. 31, 2012 Exchange	Mar. 31, 2013 Exchange Fixed maturities Holdings	Mar. 31, 2012 Exchange Fixed maturities Holdings	Mar. 31, 2013 Exchange Nonredeemable preferred stock Holdings	Mar. 31, 2013 Exchange Level 3	Mar. 31, 2012 Exchange Level 3	Mar. 31, 2013 Exchange Level 3 Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Trading securities	Mar. 31, 2012 Exchange Level 3 Trading securities	Mar. 31, 2013 Exchange Level 3 Fixed maturities Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Fixed maturities Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 States & political subdivisions Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Corporate debt securities Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Corporate debt securities Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Commercial mortgage-backed securities Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Collateralized debt obligations Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Collateralized debt obligations Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Other debt securities Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Nonredeemable preferred stock Available-for-sale securities	Mar. 31, 2012 Exchange Level 3 Nonredeemable preferred stock Available-for-sale securities	Mar. 31, 2013 Exchange Level 3 Common stock Trading securities	Mar. 31, 2012 Exchange Level 3 Common stock Trading securities	Mar. 31, 2013 Exchange Level 3 Real estate Limited partnerships	Mar. 31, 2012 Exchange Level 3 Real estate Limited partnerships	Mar. 31, 2013 Fair value measurements on a recurring basis	Mar. 31, 2013 Fair value measurements on a recurring basis Real estate Limited partnerships	Mar. 31, 2013 Fair value measurements on a recurring basis Level 3 Real estate fund	Dec. 31, 2012 Fair value measurements on a recurring basis Level 3 Real estate fund	Mar. 31, 2012 Fair value measurements on a recurring basis Level 3 Real estate fund	Mar. 31, 2013 Fair value measurements on a recurring basis Indemnity	Mar. 31, 2013 Fair value measurements on a recurring basis Exchange
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	$ 206	$ 190									$ 23	$ 21	$ 4	$ 4	$ 4	$ 4	$ 1	$ 0	$ 3	$ 4	$ 19	$ 17						$ 183	$ 169	$ 59	$ 55	$ 15	$ 12	$ 59	$ 50	$ 4	$ 43	$ 12	$ 0	$ 16	$ 29	$ 5	$ 0	$ 5	$ 15	$ 12	$ 109	$ 102	$ 99	$ 132				$ 3	$ 96
Included in earnings	3	3	(2)	2	5	1					1	0	0	0	0	0	0	0	0	0	1	0						2	3	1	0	(3)	2	1	0	0	0	0	0	1	0	0	0	0	(3)	2	4	1
Included in other comprehensive income	5	2									0	0	0	0	0	0	0	0	0	0	0	0						5	2	5	2	0	0	2	1	1	1	0	0	1	0	0	3	1	0	0	0	0
Purchases	4	1									0	0	0	0	0	0	0	0	0	0	0	0						4	1	4	0	0	0	0	0	0	0	0	0	0	0	0	4	0	0	0	0	1
Sales	(13)	(5)									(1)	0	(1)	0	(1)	0	0	0	(1)	0	0	0						(12)	(5)	(6)	(4)	(5)	0	(6)	(4)	0	(1)	0	0	(5)	(4)	0	0	0	(5)	0	(1)	(1)
Transfers in and (out) of Level 3	26	17									0	1	0	1	0	1	0	1	0	0	0	0						26	16	26	16	0	0	21	16	0	15	14	5	1	2	0	5	0	0	0	0	0
Ending balance	231	208									23	22	3	5	3	5	1	1	2	4	20	17						208	186	89	69	7	14	77	63	5	58	26	5	14	27	5	12	6	7	14	112	103	99	132				3	96
Fair value transfers due to changes in available market observable inputs
Number of real estate funds reported under fair value option																																																			4	4	4
Transfers from level 1 to level 2							0	0															6	0
Transfers from level 2 to level 1							1	0															51	0
Transfers from level 2 to level 3							0	1															39	16
Transfers from level 3 to level 2							$ 0	$ 0															$ 13	$ 0
No. of securities transferred between Level 1 and Level 2																											1
No. of securities transferred between Level 2 and Level 1										1																	5
No. of securities transferred between Level 2 and Level 3									1																7	5	1
No. of securities transferred between Level 3 and Level 2																									1