Segments Information - Revenues and Income Before Income Tax of Each Operating Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Revenues
Premiums earned related to insurance business	$ 2,977	$ 2,929	$ 8,908	$ 8,693
Investment income	383	415	1,174	1,235
Premiums earned related to reinsurance assumed		3	3	6
Net premiums earned	2,977	2,932	8,911	8,699
Corporate and other revenues	12	13	36	41
Realized investment gains, net		71	103	300
Total revenues	3,362	3,420	10,195	10,240
Insurance underwriting income	412	(71)	831	194
Reinsurance assumed underwriting income	15	11	34	26
Underwriting income before deferred policy acquisition costs	427	(60)	865	220
Increase (decrease) in deferred policy acquisition costs	(9)	13	15	70
Underwriting income (loss)	418	(47)	880	290
Total property and casualty insurance	783	357	2,003	1,485
Corporate and other loss	(56)	(62)	(173)	(188)
Realized investment gains, net		71	103	300
Income Before Federal and Foreign Income Tax	727	366	1,933	1,597
Personal insurance [Member]
Revenues
Premiums earned related to insurance business	1,009	987	3,013	2,924
Insurance underwriting income	155	(168)	374	(84)
Commercial insurance [Member]
Revenues
Premiums earned related to insurance business	1,299	1,249	3,890	3,692
Insurance underwriting income	194	8	263	(90)
Specialty insurance [Member]
Revenues
Premiums earned related to insurance business	669	693	2,005	2,077
Insurance underwriting income	63	89	194	368
Property and casualty insurance [Member]
Revenues
Investment income	373	404	1,145	1,200
Total revenues	3,350	3,336	10,056	9,899
Investment income	364	396	1,117	1,171
Other income	$ 1	$ 8	$ 6	$ 24