Business Combinations - Schedule of Business Acquisition Pro-forma financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summarized proforma income statement data:
Net interest income	$ 61,259	$ 57,329	$ 178,434	$ 168,363
Provision for (benefit from) loan losses	2,651	765	1,374	(1,728)
Noninterest income	12,288	13,902	38,517	40,537
Noninterest expense	(40,850)	(45,983)	(135,048)	(135,218)
Income before taxes	30,046	24,483	80,529	75,410
Income taxes	8,384	9,055	22,996	27,436
Net income	$ 21,662	$ 15,428	$ 57,533	$ 47,974
Basic earnings per share	$ 0.72	$ 0.51	$ 1.76	$ 1.58
Diluted earnings per share	$ 0.71	$ 0.50	$ 1.74	$ 1.56