Supplementary Insurance Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 61,005		$ 63,984		$ 55,201
Reserve for losses and loss adjustment expenses	2,097,048		2,082,679		1,985,838
Unearned premiums	642,407		532,628		463,515
Other policy claims and benefits payable
Net earned premiums	781,964		691,645		659,931
Net investment income	54,100	[1]	63,119	[1]	71,078	[1]
Losses and loss adjustment expenses incurred	497,433		476,997		421,155
Amortization of deferred policy acquisition costs	123,819	[2]	112,506	[2]	109,113	[2]
Other operating expenses	159,079	[1]	138,029	[1]	139,743	[1]
Net written premiums	833,655		753,798		653,938
Insurance Companies
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	48,294		47,298		33,273
Reserve for losses and loss adjustment expenses	1,567,045		1,517,331		1,433,556
Unearned premiums	470,425		384,596		318,119
Other policy claims and benefits payable
Net earned premiums	571,439		472,463		438,851
Net investment income	46,549	[1]	54,164	[1]	62,792	[1]
Losses and loss adjustment expenses incurred	417,082		341,625		280,120
Amortization of deferred policy acquisition costs	81,370	[2]	64,165	[2]	59,122	[2]
Other operating expenses	113,625	[1]	101,517	[1]	106,631	[1]
Net written premiums	622,956		542,391		429,355
Lloyd's Operations
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	12,711		16,686		21,928
Reserve for losses and loss adjustment expenses	530,003		565,348		552,282
Unearned premiums	171,982		148,032		145,396
Other policy claims and benefits payable
Net earned premiums	210,525		219,182		221,080
Net investment income	7,551	[1]	8,955	[1]	8,286	[1]
Losses and loss adjustment expenses incurred	80,351		135,372		141,035
Amortization of deferred policy acquisition costs	42,449	[2]	48,341	[2]	49,991	[2]
Other operating expenses	45,454	[1]	36,512	[1]	33,112	[1]
Net written premiums	$ 210,699		$ 211,407		$ 224,583

[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. A portion of these costs is eliminated in consolidation.