Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Investment Income									$ 82,770	$ 72,068	$ 67,166
Investment expenses									(3,319)	(3,350)	(2,998)
Net investment income	$ 20,107	$ 19,875	$ 19,875	$ 19,594	$ 18,499	$ 17,371	$ 16,595	$ 16,253	79,451	68,718	64,168
Fixed maturities
Net Investment Income [Line Items]
Investment Income									67,772	61,572	57,219
Equity Securities
Net Investment Income [Line Items]
Investment Income									14,271	9,813	9,036
Short-term investments
Net Investment Income [Line Items]
Investment Income									$ 727	$ 683	$ 911