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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on investments	$ (82)	$ (1,111)	$ (23)	$ (1,142)
Income before income taxes	22,169	20,930	45,978	40,344
Income tax expense	(4,279)	(3,974)	(8,688)	(7,219)
Net income	17,890	16,956	37,290	33,125
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	26	(762)	(105)	(466)
Income before income taxes	26	(762)	(105)	(466)
Income tax expense	(5)	160	22	44
Net income	$ 21	$ (602)	$ (83)	$ (422)