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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains (losses) on available-for-sale securities	$ (65)	$ (329)	$ (1,111)	$ (31)	$ 114	$ (192)	$ (388)	$ (181)	$ (1,536)	$ (647)	$ (494)
Income before income taxes	22,777	24,460	20,930	19,414	17,800	23,556	22,151	18,733	87,581	82,240	112,821
Income tax expense									(15,949)	(36,009)	(34,956)
Net income	$ 18,806	$ 19,701	$ 16,956	$ 16,169	$ 649	$ 16,577	$ 15,481	$ 13,524	71,632	46,231	77,865
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									(390)	1,106	(234)
Income before income taxes									(390)	1,106	(234)
Income tax expense									82	(387)	82
Net income									$ (308)	$ 719	$ (152)