Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments	$ (332)	$ 309	$ 163	$ 992	$ (53)	$ (4)	$ (82)	$ 59	$ 1,132	$ (80)	$ (1,536)
Income before income taxes	35,618	28,669	29,391	13,241	42,926	26,613	22,169	23,809	106,919	115,517	87,581
Income tax expense									(20,317)	(22,827)	(15,949)
Net income	$ 28,501	$ 23,353	$ 23,948	$ 10,800	$ 34,014	$ 21,386	$ 17,890	$ 19,400	86,602	92,690	71,632
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									925	(841)	(390)
Income before income taxes									925	(841)	(390)
Income tax expense									(195)	177	82
Net income									$ 730	$ (664)	$ (308)