Segment Information (Detail) - Net premiums earned by line of business (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
P&C Operations:
P&C Operations	$ 1,357,898		$ 1,271,696
Life Operations:
Life Operations	78,520		89,687
Total	1,436,418		1,361,383
Insurance Segment [Member] | Other Life [Member]
Life Operations:
Life Operations	0		0
Insurance Segment [Member] | Annuity [Member]
Life Operations:
Life Operations	0		0
Insurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
P&C Operations	323,821		315,643
Insurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
P&C Operations	167,357		156,704
Insurance Segment [Member] | Property Catastrophe [Member]
P&C Operations:
P&C Operations	0		0
Insurance Segment [Member] | Other Property [Member]
P&C Operations:
P&C Operations	133,419		106,921
Insurance Segment [Member] | Marine, Energy, Aviation and Satellite [Member]
P&C Operations:
P&C Operations	120,100		125,536
Insurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
P&C Operations	184,034	[1]	168,666	[1]
Insurance Segment [Member] | Other Lines [Member]
P&C Operations:
P&C Operations	5,325	[2]	2,450	[2]
Insurance Segment [Member]
P&C Operations:
P&C Operations	934,056		875,920
Life Operations:
Life Operations	0		0
Total	934,056		875,920
Reinsurance Segment [Member] | Other Life [Member]
Life Operations:
Life Operations	0		0
Reinsurance Segment [Member] | Annuity [Member]
Life Operations:
Life Operations	0		0
Reinsurance Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
P&C Operations	54,908		53,718
Reinsurance Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
P&C Operations	73,238		51,043
Reinsurance Segment [Member] | Property Catastrophe [Member]
P&C Operations:
P&C Operations	102,925		94,564
Reinsurance Segment [Member] | Other Property [Member]
P&C Operations:
P&C Operations	135,743		136,243
Reinsurance Segment [Member] | Marine, Energy, Aviation and Satellite [Member]
P&C Operations:
P&C Operations	35,743		38,428
Reinsurance Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
P&C Operations	0	[1]	0	[1]
Reinsurance Segment [Member] | Other Lines [Member]
P&C Operations:
P&C Operations	21,285	[2]	21,780	[2]
Reinsurance Segment [Member]
P&C Operations:
P&C Operations	423,842		395,776
Life Operations:
Life Operations	0		0
Total	423,842		395,776
Life Operations Segment [Member] | Other Life [Member]
Life Operations:
Life Operations	47,063		56,764
Life Operations Segment [Member] | Annuity [Member]
Life Operations:
Life Operations	31,457		32,923
Life Operations Segment [Member] | Casualty Professional Lines [Member]
P&C Operations:
P&C Operations	0		0
Life Operations Segment [Member] | Casualty Other Lines [Member]
P&C Operations:
P&C Operations	0		0
Life Operations Segment [Member] | Property Catastrophe [Member]
P&C Operations:
P&C Operations	0		0
Life Operations Segment [Member] | Other Property [Member]
P&C Operations:
P&C Operations	0		0
Life Operations Segment [Member] | Marine, Energy, Aviation and Satellite [Member]
P&C Operations:
P&C Operations	0		0
Life Operations Segment [Member] | Other Speciality Lines [Member]
P&C Operations:
P&C Operations	0	[1]	0	[1]
Life Operations Segment [Member] | Other Lines [Member]
P&C Operations:
P&C Operations	0	[2]	0	[2]
Life Operations Segment [Member]
P&C Operations:
P&C Operations	0		0
Life Operations:
Life Operations	78,520		89,687
Total	78,520		89,687
Other Life [Member]
Life Operations:
Life Operations	47,063		56,764
Annuity [Member]
Life Operations:
Life Operations	31,457		32,923
Casualty Professional Lines [Member]
P&C Operations:
P&C Operations	378,729		369,361
Casualty Other Lines [Member]
P&C Operations:
P&C Operations	240,595		207,747
Property Catastrophe [Member]
P&C Operations:
P&C Operations	102,925		94,564
Other Property [Member]
P&C Operations:
P&C Operations	269,162		243,164
Marine, Energy, Aviation and Satellite [Member]
P&C Operations:
P&C Operations	155,843		163,964
Other Speciality Lines [Member]
P&C Operations:
P&C Operations	184,034	[1]	168,666	[1]
Other Lines [Member]
P&C Operations:
P&C Operations	$ 26,610	[2]	$ 24,230	[2]

[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, structured indemnity and other lines.