Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains on investments	$ 1,179	$ 163	$ 1,498	$ 1,155
Income before income taxes	29,171	29,391	52,796	42,632
Income tax expense	(5,404)	(5,443)	(9,717)	(7,884)
Net income	23,767	23,948	43,079	34,748
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 on investments	1,580	133	1,976	514
Income before income taxes	1,580	133	1,976	514
Income tax expense	(332)	(28)	(415)	(108)
Net income	$ 1,248	$ 105	$ 1,561	$ 406