Supplementary Information Concerning Property-Casualty Insurance Operations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [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
Discount, if any, deducted	0		0		0
Unearned premiums	766,167		714,606		642,407
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 related to Current year	601,041		520,227		542,724
Increase (decrease) in estimated losses and LAE for claims occurring in prior years	(55,812)		(1,266)		(45,291)
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

[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.