Segment Information (Detail) - Net premiums earned by line of business (USD $) In Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
P&C Operations:
Premiums Earned Net Property And Casualty	$ 1,306,125		$ 1,216,313		$ 2,577,821		$ 2,479,914
Life Operations:
Premiums Earned Net Life	92,214		86,448		181,901		191,332
Total	1,398,339		1,302,761		2,759,722		2,671,246
Life Operations [Member] | Other Life [Member]
Life Operations:
Premiums Earned Net Life	58,246		55,879		115,010		129,332
Other Life [Member]
Life Operations:
Premiums Earned Net Life	58,246		55,879		115,010		129,332
Life Operations [Member] | Annuity [Member]
Life Operations:
Premiums Earned Net Life	33,968		30,569		66,891		62,000
Annuity [Member]
Life Operations:
Premiums Earned Net Life	33,968		30,569		66,891		62,000
Insurance Segment [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	907,443		868,666		1,783,363		1,765,677
Life Operations:
Total	907,443		868,666		1,783,363		1,765,677
Insurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	322,654
Insurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	179,495		141,353		338,325		298,719
Insurance Segment [Member] | Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	116,829		108,016		223,693		205,555
Insurance Segment [Member] | Marine Energy Aviationand Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	129,008		133,720		255,075		274,490
Insurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	157,386	[1]	151,016	[1]	323,452	[1]	312,644	[1]
Insurance Segment [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	1,064	[2]	3,389	[2]	2,416	[2]	3,353	[2]
Insurance Segment [Member] | Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	1,007		3,103		2,105		5,789
Insurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			328,069		638,297		665,127
Reinsurance Segment [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	398,682		347,647		794,458		714,237
Life Operations:
Total	398,682		347,647		794,458		714,237
Reinsurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	51,914
Reinsurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	68,920		51,320		119,963		115,453
Reinsurance Segment [Member] | Property Catastrophe [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	94,363		80,876		188,927		165,511
Reinsurance Segment [Member] | Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	135,255		115,425		271,497		228,688
Reinsurance Segment [Member] | Marine Energy Aviationand Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	30,435		22,452		68,863		42,641
Reinsurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty		[1]		[1]		[1]		[1]
Reinsurance Segment [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	17,903	[2]	25,847	[2]	43,452	[2]	54,844	[2]
Reinsurance Segment [Member] | Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	(108)		62		(3,877)		(285)
Reinsurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			51,665		105,633		107,385
Life Operations [Member]
Life Operations:
Premiums Earned Net Life	92,214		86,448		181,901		191,332
Total	92,214		86,448		181,901		191,332
Life Operations [Member] | Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty				[1]		[1]		[1]
Life Operations [Member] | Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty		[2]		[2]		[2]		[2]
Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	374,568
Casualty Professional Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty			379,734		743,930		772,512
Casualty Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	248,415		192,673		458,288		414,172
Property Catastrophe [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	94,363		80,876		188,927		165,511
Other Property [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	252,084		223,441		495,190		434,243
Marine Energy Aviationand Satellite [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	159,443		156,172		323,938		317,131
Other Speciality Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	157,386	[1]	151,016	[1]	323,452	[1]	312,644	[1]
Other Lines [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	18,967	[2]	29,236	[2]	45,868	[2]	58,197	[2]
Structured Indemnity [Member]
P&C Operations:
Premiums Earned Net Property And Casualty	$ 899		$ 3,165		$ (1,772)		$ 5,504

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

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