Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Investment Income [Line Items]
Investment Income									$ 72,068	$ 67,166	$ 59,507
Investment expenses									(3,350)	(2,998)	(3,256)
Net investment income	$ 18,499	$ 17,371	$ 16,595	$ 16,253	$ 16,071	$ 15,839	$ 15,648	$ 16,610	68,718	64,168	56,251
Fixed maturities
Net Investment Income [Line Items]
Investment Income									61,572	57,219	53,898
Equity Securities
Net Investment Income [Line Items]
Investment Income									9,813	9,036	4,835
Short-term investments
Net Investment Income [Line Items]
Investment Income									$ 683	$ 911	$ 774