Supplementary Information Concerning Property-Casualty Insurance Operations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplemental Information for Property, Casualty Insurance Underwriters [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
Discount, if any, deducted
Unearned premiums	642,407		532,628		463,515
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 related to expenses incurred Current year	542,724		474,852		434,957
Losses and loss adjustment related to expenses incurred Prior year	(45,291)		2,145		(13,802)
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

[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.