Summary of Activity in Reserve for Losses and Loss Adjustment Expenses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserves for losses and LAE at beginning of year	$ 1,222,633	$ 1,216,909	$ 1,237,234
Provision for losses and LAE for claims occurring in the 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)
Incurred losses and LAE	545,229	518,961	497,433
Losses and LAE paid for claims occurring during:
Current year	(164,199)	(147,758)	(110,373)
Prior years	(295,527)	(365,479)	(407,385)
Losses and LAE payments	(459,726)	(513,237)	(517,758)
Net reserves for losses and LAE at end of year	1,308,136	1,222,633	1,216,909
Reinsurance recoverables on unpaid losses and LAE	851,498	822,438	880,139
Gross reserves for losses and LAE at end of year	$ 2,159,634	$ 2,045,071	$ 2,097,048