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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 20,930	$ 22,151	$ 40,344	$ 40,884
Income tax expense	(3,974)	(6,670)	(7,219)	(11,879)
Net income	16,956	15,481	33,125	29,005
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	(762)	126	(466)	286
Income before income taxes	(762)	126	(466)	286
Income tax expense	160	(44)	44	(100)
Net income	$ (602)	$ 82	$ (422)	$ 186