Supplementary Insurance Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 63,984		$ 55,201		$ 56,575
Reserve for losses and loss adjustment expenses	2,082,679		1,985,838		1,920,286
Unearned premiums	532,628		463,515		475,171
Other policy claims and benefits payable
Net earned premiums	691,645		659,931		683,363
Net investment income	63,119	[1]	71,078	[1]	74,946	[1]
Losses and loss adjustment expenses incurred	476,997		421,155		435,998
Amortization of deferred policy acquisition costs	112,506	[2]	109,113	[2]	99,676	[2]
Other operating expenses	138,029	[1]	139,743	[1]	132,697	[1]
Net written premiums	753,798		653,938		701,255
Insurance Companies
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	47,298		33,273		34,872
Reserve for losses and loss adjustment expenses	1,517,331		1,433,556		1,395,876
Unearned premiums	384,596		318,119		334,798
Other policy claims and benefits payable
Net earned premiums	472,463		438,851		479,121
Net investment income	54,164	[1]	62,792	[1]	65,717	[1]
Losses and loss adjustment expenses incurred	341,625		280,120		304,672
Amortization of deferred policy acquisition costs	64,165	[2]	59,122	[2]	61,949	[2]
Other operating expenses	101,517	[1]	106,631	[1]	104,801	[1]
Net written premiums	542,391		429,355		477,673
Lloyd's Operations
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	16,686		21,928		21,703
Reserve for losses and loss adjustment expenses	565,348		552,282		524,410
Unearned premiums	148,032		145,396		140,373
Other policy claims and benefits payable
Net earned premiums	219,182		221,080		204,242
Net investment income	8,955	[1]	8,286	[1]	9,229	[1]
Losses and loss adjustment expenses incurred	135,372		141,035		131,326
Amortization of deferred policy acquisition costs	48,341	[2]	49,991	[2]	37,727	[2]
Other operating expenses	36,512	[1]	33,112	[1]	27,896	[1]
Net written premiums	$ 211,407		$ 224,583		$ 223,582

[1] Net investment income and Other operating expenses reflect only such amounts attributable to the Company's insurance operations.

[2] Amortization of deferred policy acquisition costs reflects only such amounts attributable to the Company's insurance operations.