Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains (losses) on available-for-sale securities	$ 2,200	$ (400)
Income before income taxes	23,809	19,414
Income tax expense	(4,409)	(3,245)
Net income	19,400	16,169
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]
Unrealized gains (losses) on available-for-sale securities	(14)	17
Income before income taxes	(14)	17
Income tax expense	3	(57)
Net income	$ (11)	$ (40)