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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments	$ 163	$ (82)	$ 1,155	$ (23)
Income before income taxes	29,391	22,169	42,632	45,978
Income tax expense	(5,443)	(4,279)	(7,884)	(8,688)
Net income	23,948	17,890	34,748	37,290
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	133	26	514	(105)
Income before income taxes	133	26	514	(105)
Income tax expense	(28)	(5)	(108)	22
Net income	$ 105	$ 21	$ 406	$ (83)