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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on investments	$ (53)	$ (4)	$ (82)	$ 59	$ (65)	$ (329)	$ (1,111)	$ (31)	$ (80)	$ (1,536)	$ (647)
Income before income taxes	42,926	26,613	22,169	23,809	22,777	24,460	20,930	19,414	115,517	87,581	82,240
Income tax expense									(22,827)	(15,949)	(36,009)
Net income	$ 34,014	$ 21,386	$ 17,890	$ 19,400	$ 18,806	$ 19,701	$ 16,956	$ 16,169	92,690	71,632	46,231
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 losses on investments									(841)	(390)	1,106
Income before income taxes									(841)	(390)	1,106
Income tax expense									177	82	(387)
Net income									$ (664)	$ (308)	$ 719