Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	$ 15,790	$ 17,003	$ 49,161	$ 53,221
Investment expenses	(2,193)	(744)	(8,529)	(2,149)
Net investment income	13,597	16,259	40,632	51,072
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	14,364	15,856	44,779	49,892
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	989	942	2,930	2,584
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	$ 437	$ 205	$ 1,452	$ 745