Fair Value (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Level 3
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	$ 231	$ 208	$ 206	$ 190
Included in earnings	5	5	6	9
Included in other comprehensive income	(3)	(1)	1	0
Purchases	1	1	5	2
Sales	(13)	(5)	(20)	(10)
Transfers in and (out) of Level 3	(23)	0	0	17
Ending balance	198	208	198	208
Level 3 | Net realized investment gains losses
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Included in earnings	3	(1)	(2)	2
Level 3 | Equity in earnings of limited partnerships
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Included in earnings	2	6	8	7
Indemnity
Fair value transfers due to changes in available market observable inputs
Transfers from level 1 to level 2	0	1	0	1
Transfers from level 2 to level 1	0	0	1	0
Transfers from level 2 to level 3	0	0	0	1
Transfers from level 3 to level 2	0	0	0	0
Indemnity | Fixed maturities
Fair value transfers due to changes in available market observable inputs
No. of securities transferred between Level 2 and Level 3				1
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		1		1
No. of securities transferred between Level 2 and Level 1			1
Indemnity | Level 3
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	23	22	23	21
Included in earnings	0	1	1	1
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	(1)	0	(2)	0
Transfers in and (out) of Level 3	0	0	0	1
Ending balance	22	23	22	23
Indemnity | Level 3 | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	3	5	4	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	(1)	0	(2)	0
Transfers in and (out) of Level 3	0	0	0	1
Ending balance	2	5	2	5
Indemnity | Level 3 | Fixed maturities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	3	5	4	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	(1)	0	(2)	0
Transfers in and (out) of Level 3	0	0	0	1
Ending balance	2	5	2	5
Indemnity | Level 3 | Corporate debt securities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	1	1	1	0
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	0
Transfers in and (out) of Level 3	0	0	0	1
Ending balance	1	1	1	1
Indemnity | Level 3 | Collateralized debt obligations | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	2	4	3	4
Included in earnings	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	(1)	0	(2)	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	1	4	1	4
Indemnity | Level 3 | Real estate | Limited partnerships
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	20	17	19	17
Included in earnings	0	1	1	1
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	20	18	20	18
Exchange
Fair value transfers due to changes in available market observable inputs
Transfers from level 1 to level 2	0	20	5	20
Transfers from level 2 to level 1	0	28	44	27
Transfers from level 2 to level 3	0	0	18	16
Transfers from level 3 to level 2	23	0	18	0
Exchange | Fixed maturities
Fair value transfers due to changes in available market observable inputs
No. of securities transferred between Level 2 and Level 3			5	6
No. of securities transferred between Level 3 and Level 2	3		2
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		4	1	4
No. of securities transferred between Level 2 and Level 1		3	5	3
Exchange | Level 3
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	208	186	183	169
Included in earnings	5	4	5	8
Included in other comprehensive income	(3)	(1)	1	0
Purchases	1	1	5	2
Sales	(12)	(5)	(18)	(10)
Transfers in and (out) of Level 3	(23)	0	0	16
Ending balance	176	185	176	185
Exchange | Level 3 | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	89	69	59	55
Included in earnings	3	(1)	1	0
Included in other comprehensive income	(3)	(1)	1	0
Purchases	1	1	5	1
Sales	(11)	(4)	(10)	(8)
Transfers in and (out) of Level 3	(23)	0	0	16
Ending balance	56	64	56	64
Exchange | Level 3 | Trading securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	7	14	15	12
Included in earnings	0	0	(3)	2
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(5)	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	7	14	7	14
Exchange | Level 3 | Fixed maturities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	77	63	59	50
Included in earnings	1	0	1	0
Included in other comprehensive income	(2)	(1)	(2)	0
Purchases	1	1	1	1
Sales	(5)	(4)	(10)	(8)
Transfers in and (out) of Level 3	(23)	0	0	16
Ending balance	49	59	49	59
Exchange | Level 3 | States & political subdivisions | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance		5		4
Included in earnings		0		0
Included in other comprehensive income		(1)		0
Purchases		0		0
Sales		0		0
Transfers in and (out) of Level 3		0		0
Ending balance		4		4
Exchange | Level 3 | Corporate debt securities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	58	26	43	12
Included in earnings	0	0	0	0
Included in other comprehensive income	(1)	0	(1)	0
Purchases	1	1	1	1
Sales	(2)	(4)	(2)	(4)
Transfers in and (out) of Level 3	(23)	0	(8)	14
Ending balance	33	23	33	23
Exchange | Level 3 | Commercial mortgage-backed securities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	5		0
Included in earnings	0		0
Included in other comprehensive income	0		(1)
Purchases	0		0
Sales	(1)		(1)
Transfers in and (out) of Level 3	0		6
Ending balance	4		4
Exchange | Level 3 | Collateralized debt obligations | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	14	27	16	29
Included in earnings	1	0	1	0
Included in other comprehensive income	(1)	0	0	0
Purchases	0	0	0	0
Sales	(2)	0	(7)	(4)
Transfers in and (out) of Level 3	0	0	2	2
Ending balance	12	27	12	27
Exchange | Level 3 | Other debt securities | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance		5		5
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		0
Ending balance		5		5
Exchange | Level 3 | Nonredeemable preferred stock | Available-for-sale securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	12	6	0	5
Included in earnings	2	(1)	0	0
Included in other comprehensive income	(1)	0	3	0
Purchases	0	0	4	0
Sales	(6)	0	0	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	7	5	7	5
Exchange | Level 3 | Common stock | Trading securities
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	7	14	15	12
Included in earnings	0	0	(3)	2
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(5)	0
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	7	14	7	14
Exchange | Level 3 | Real estate | Limited partnerships
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Beginning balance	112	103	109	102
Included in earnings	2	5	7	6
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	1
Sales	(1)	(1)	(3)	(2)
Transfers in and (out) of Level 3	0	0	0	0
Ending balance	113	107	113	107
Fair value measurements on a recurring basis
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Ending balance	65		65
Fair value measurements on a recurring basis | Real estate | Limited partnerships
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Ending balance	133		133
Fair value measurements on a recurring basis | Indemnity
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Ending balance	2		2
Fair value measurements on a recurring basis | Exchange
Roll forward of consolidated level 3 fair value measurements on a recurring basis
Ending balance	$ 63		$ 63