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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments	$ (8)	$ 309	$ 1,490	$ 1,464
Income before income taxes	24,721	28,669	77,517	71,301
Income tax expense	(5,585)	(5,316)	(15,302)	(13,200)
Net income	19,136	23,353	62,215	58,101
Unrealized Gains On Debt Securities, Net Of Tax [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	331	737	2,307	780
Income before income taxes	331	737	2,307	780
Income tax expense	(69)	(155)	(484)	(164)
Net income	$ 262	$ 582	$ 1,823	$ 616