Net Investment Income (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
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									$ 64,634	$ 71,095	$ 73,989
Investment expenses									(10,386)	(7,595)	(2,327)
Net investment income	13,616	13,597	15,777	11,258	12,428	16,259	17,429	17,384	54,248	63,500	71,662
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									58,995	65,060	69,996
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									3,945	5,071	3,028
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income									$ 1,694	$ 964	$ 965