Supplementary Insurance Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 79,452		$ 67,007		$ 61,005
Reserve for losses and loss adjustment expenses	2,159,634		2,045,071		2,097,048
Unearned Premiums	766,167		714,606		642,407
Other policy claims and benefits payable	0		0		0
Net earned premiums	935,895		841,939		781,964
Net investment income	64,092	[1]	56,243	[1]	54,100	[1]
Losses and loss adjustment expenses incurred	545,229		518,961		497,433
Amortization of deferred policy acquisition costs	127,695	[2]	115,842	[2]	123,819	[2]
Other operating expenses	196,825	[1]	164,434	[1]	159,079	[1]
Net written premiums	1,000,138		887,922		833,655
Insurance Companies
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	63,200		54,984		48,294
Reserve for losses and loss adjustment expenses	1,645,984		1,523,175		1,567,045
Unearned Premiums	579,331		536,303		470,425
Other policy claims and benefits payable	0		0		0
Net earned premiums	704,574		639,338		571,439
Net investment income	56,714	[1]	49,083	[1]	46,549	[1]
Losses and loss adjustment expenses incurred	434,396		415,413		417,082
Amortization of deferred policy acquisition costs	85,137	[2]	81,132	[2]	81,370	[2]
Other operating expenses	138,675	[1]	119,920	[1]	113,625	[1]
Net written premiums	752,773		680,008		622,956
Lloyd's
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	16,252		12,023		12,711
Reserve for losses and loss adjustment expenses	513,650		521,896		530,003
Unearned Premiums	186,836		178,303		171,982
Other policy claims and benefits payable	0		0		0
Net earned premiums	231,321		202,601		210,525
Net investment income	7,378	[1]	7,160	[1]	7,551	[1]
Losses and loss adjustment expenses incurred	110,833		103,548		80,351
Amortization of deferred policy acquisition costs	42,558	[2]	34,710	[2]	42,449	[2]
Other operating expenses	58,150	[1]	44,514	[1]	45,454	[1]
Net written premiums	$ 247,365		$ 207,914		$ 210,699

[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.