Summary of Earned Premium (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct									$ 972,844	$ 912,020	$ 925,935
Assumed									205,759	125,797	72,643
Ceded									(396,639)	(346,172)	(338,647)
Net earned premiums	$ 201,566	$ 201,262	$ 196,017	$ 183,119	$ 191,757	$ 173,633	$ 173,777	$ 152,478	$ 781,964	$ 691,645	$ 659,931