Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 24,460	$ 23,556	$ 64,804	$ 64,440
Income tax expense	(4,759)	(6,979)	(11,978)	(18,858)
Net income	19,701	16,577	52,826	45,582
Unrealized Gains 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	(32)	312	(445)	780
Income before income taxes	(32)	312	(445)	780
Income tax expense	7	(109)	40	(273)
Net income	$ (25)	$ 203	$ (405)	$ 507