Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	$ 16,701	$ 17,977	$ 33,371	$ 36,218
Investment expenses	(924)	(548)	(6,336)	(1,405)
Net investment income	15,777	17,429	27,035	34,813
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	15,006	16,844	30,416	34,034
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	994	859	1,941	1,642
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income	$ 701	$ 274	$ 1,014	$ 542