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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 23,556	$ 26,027	$ 64,440	$ 85,037
Income tax expense	(6,979)	(8,131)	(18,858)	(26,245)
Net income	16,577	17,896	45,582	58,792
Changes in Net Unrealized Gains (Losses) on Investment Securities [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on available-for-sale securities	312	191	780	426
Income before income taxes	312	191	780	426
Income tax expense	(109)	(67)	(273)	(149)
Net income	$ 203	$ 124	$ 507	$ 277