Summary of Activity in Reserve for Losses and Loss Adjustment Expenses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserves for losses and LAE at beginning of year	$ 1,237,234	$ 1,142,542	$ 1,112,934
Provision for losses and LAE for claims occurring in the current year	542,724	474,852	434,957
Increase (decrease) in estimated losses and LAE for claims occurring in prior years	(45,291)	2,145	(13,802)
Incurred losses and LAE	497,433	476,997	421,155
Losses and LAE paid for claims occurring during:
Current year	(110,373)	(73,242)	(76,982)
Prior years	(407,385)	(309,063)	(314,565)
Losses and LAE payments	(517,758)	(382,305)	(391,547)
Net reserves for losses and LAE at end of year	1,216,909	1,237,234	1,142,542
Reinsurance recoverables on unpaid losses and LAE	880,139	845,445	843,296
Gross reserves for losses and LAE at end of year	$ 2,097,048	$ 2,082,679	$ 1,985,838