Segment Information (Detail) - Net premiums earned by line of business (USD $) In Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
P&C Operations:
Premiums Earned Net Property And Casualty	$ 1,362,532		$ 1,268,741		$ 3,940,353		$ 3,748,655
Life Operations:
Premiums Earned Net Life	90,794		96,586		272,695		287,918
Total	1,453,326		1,365,327		4,213,048		4,036,573
Insurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	179,723		147,152	[1]	518,048		445,871	[1]
Reinsurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	64,275		55,534	[1]	184,238		170,987	[1]
Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	243,998		202,686		702,286		616,858
Insurance Segment [Member] | Property Catastrophe [Member]
P&C Operations:
Premiums Earned Net Property And Casualty				[1]				[1]
Reinsurance Segment [Member] | Property Catastrophe [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	97,158		77,234	[1]	286,085		242,745	[1]
Property Catastrophe [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	97,158		77,234		286,085		242,745
Insurance Segment [Member] | Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	119,953		102,928	[1]	343,646		308,483	[1]
Reinsurance Segment [Member] | Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	161,585		153,275	[1]	433,082		381,963	[1]
Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	281,538		256,203		776,728		690,446
Insurance Segment [Member] | Marine Energy Aviation and Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	138,884		133,694	[1]	393,959		408,184	[1]
Reinsurance Segment [Member] | Marine Energy Aviation and Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	32,162		24,166	[1]	101,025		66,807	[1]
Marine Energy Aviation and Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	171,046		157,860		494,984		474,991
Insurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	171,514	[2]	151,742	[1],[2]	494,966	[2]	464,386	[1],[2]
Reinsurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty		[2]		[1],[2]		[2]		[1],[2]
Life Operations [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty		[2]		[2]		[2]		[2]
Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	171,514	[2]	151,742	[2]	494,966	[2]	464,386	[2]
Insurance Segment [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	587	[3]	2,723	[1],[3]	3,003	[3]	6,076	[1],[3]
Reinsurance Segment [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	25,341	[3]	27,180	[1],[3]	68,793	[3]	82,024	[1],[3]
Life Operations [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty		[3]		[3]		[3]		[3]
Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	25,928	[3]	29,903	[3]	71,796	[3]	88,100	[3]
Insurance Segment [Member] | Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	304		7,765	[1]	2,409		13,554	[1]
Reinsurance Segment [Member] | Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			954	[1]	(3,877)		669	[1]
Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	304		8,719		(1,468)		14,223
Insurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			326,144	[1]	957,399		991,271	[1]
Insurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	319,102
Reinsurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			58,250	[1]	157,577		165,635	[1]
Reinsurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	51,944
Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			384,394		1,114,976		1,156,906
Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	371,046
Insurance Segment [Member] | Other Life [Member]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Reinsurance Segment [Member] | Other Life [Member]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Life Operations [Member] | Other Life [Member]
Life Operations:
Premiums Earned Net Life	57,452		64,208		172,463		193,540
Other Life [Member]
Life Operations:
Premiums Earned Net Life	57,452		64,208		172,463		193,540
Insurance Segment [Member] | Annuity [Member]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Reinsurance Segment [Member] | Annuity [Member]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Life Operations [Member] | Annuity [Member]
Life Operations:
Premiums Earned Net Life	33,342		32,378		100,232		94,378
Annuity [Member]
Life Operations:
Premiums Earned Net Life	33,342		32,378		100,232		94,378
Insurance Segment [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	930,067		872,148	[1]	2,713,430		2,637,825	[1]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Total	930,067		872,148	[1]	2,713,430		2,637,825	[1]
Reinsurance Segment [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	432,465		396,593	[1]	1,226,923		1,110,830	[1]
Life Operations:
Premiums Earned Net Life				[1]				[1]
Total	432,465		396,593	[1]	1,226,923		1,110,830	[1]
Life Operations [Member]
Life Operations:
Premiums Earned Net Life	90,794		96,586		272,695		287,918
Total	$ 90,794		$ 96,586		$ 272,695		$ 287,918

[1] Certain reclassifications have been made to conform to current year presentation.

[2] Other specialty lines within the Insurance segment includes: environmental, programs, equine, warranty, specie, middle markets and excess and surplus lines.

[3] Other includes credit and surety, whole account contracts and other lines.