Summary of Activity in Reserve for Losses and LAE (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserves for losses and LAE at beginning of year	$ 1,142,542	$ 1,112,934	$ 999,871
Provision for losses and LAE for claims occurring in the current year	474,852	434,957	444,939
Increase (decrease) in estimated losses and LAE for claims occurring in prior years	2,145	(13,802)	(8,941)
Incurred losses and LAE	476,997	421,155	435,998
Losses and LAE paid for claims occurring during:
Current year	(73,242)	(76,982)	(59,412)
Prior years	(309,063)	(314,565)	(263,523)
Losses and LAE payments	(382,305)	(391,547)	(322,935)
Net reserves for losses and LAE at end of year	1,237,234	1,142,542	1,112,934
Reinsurance recoverables on unpaid losses and LAE	845,445	843,296	807,352
Gross reserves for losses and LAE at end of year	$ 2,082,679	$ 1,985,838	$ 1,920,286