Summary of Earned Premium (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct									$ 912,020	$ 925,935	$ 977,170
Assumed									125,797	72,643	78,932
Ceded									(346,172)	(338,647)	(372,739)
Net earned premiums	$ 191,757	$ 173,633	$ 173,777	$ 152,478	$ 166,158	$ 168,233	$ 161,471	$ 164,069	$ 691,645	$ 659,931	$ 683,363