Net Investment Income (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
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									$ 71,095	$ 73,989	$ 78,054
Investment expenses									(7,595)	(2,327)	(2,542)
Net investment income	12,428	16,259	17,429	17,384	17,998	17,839	17,853	17,972	63,500	71,662	75,512
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									65,060	69,996	74,779
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									5,071	3,028	2,464
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									$ 964	$ 965	$ 811