Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Investment Income									$ 106,250	$ 92,956	$ 82,770
Investment Expenses									(3,137)	(3,663)	(3,319)
Net Investment Income	$ 29,159	$ 25,651	$ 24,601	$ 23,702	$ 22,982	$ 22,598	$ 22,265	$ 21,448	103,113	89,293	79,451
Fixed Maturities
Net Investment Income [Line Items]
Investment Income									90,101	76,784	67,772
Equity Securities
Net Investment Income [Line Items]
Investment Income									13,807	15,108	14,271
Short-Term Investments, Cash & Cash Equivalents
Net Investment Income [Line Items]
Investment Income									1,676	428	317
Other Invested Assets
Net Investment Income [Line Items]
Investment Income									$ 666	$ 636	$ 410