Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains on investments	$ 319	$ 992
Income before income taxes	23,625	13,241
Income tax expense	(4,313)	(2,441)
Net income	19,312	10,800
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	396	438
Income before income taxes	396	438
Income tax expense	(83)	(92)
Net income	$ 313	$ 346