Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 19,414	$ 18,733
Income tax expense	(3,245)	(5,209)
Net income	16,169	13,524
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 on available-for- sale securities	17	40
Income before income taxes	17	40
Income tax expense	(57)	(14)
Net income	$ (40)	$ 26