Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments	$ 309	$ (4)	$ 1,464	$ (27)
Income before income taxes	28,669	26,613	71,301	72,591
Income tax expense	(5,316)	(5,227)	(13,200)	(13,915)
Net income	23,353	21,386	58,101	58,676
Unrealized Gains (losses) on Available-for-Sale Securities [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments	737	(10)	780	(764)
Income before income taxes	737	(10)	780	(764)
Income tax expense	(155)	2	(164)	161
Net income	$ 582	$ (8)	$ 616	$ (603)