Supplementary Information Concerning Property-Casualty Insurance Operations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Supplemental Information for Property, Casualty Insurance Underwriters [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
Discount, if any, deducted
Unearned premiums	532,628		463,515		475,171
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 related to expenses incurred Current year	474,852		434,957		444,939
Losses and loss adjustment related to expenses incurred Prior year	2,145		(13,802)		(8,941)
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

[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.