Recent Developments Including Mergers and Acquisitions - Additional Information (Detail) $ in Thousands			3 Months Ended
	Jan. 11, 2018 USD ($) Location	Jul. 31, 2017 shares	Mar. 31, 2018 USD ($) Community	Dec. 31, 2017 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Goodwill			$ 79,796	$ 54,042
Number of additional banking communities | Community			5
Common Stock [Member]
Business Combination Separately Recognized Transactions [Line Items]
Stock split description			On July 31, 2017, the Company completed a two-for-one stock split of the Company’s outstanding shares of common stock.
Stock split conversion ratio		2
Stock payable in form of dividend, payable date		Jul. 31, 2017
Stock payable in form of dividend, record date		Jul. 17, 2017
Additional shares received by stockholders for each share held | shares		1
First Bank & Trust Company [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition	Jan. 11, 2018
Total assets acquired	$ 290,000
Loans	247,000
Deposits	251,000
Equity capital	36,000
Intangibles assets, net	6,300
Goodwill	$ 19,100
Fair value estimates period	1 year
First Bank of Chandler [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition	Jan. 11, 2018
Total assets acquired	$ 88,000
Loans	66,000
Deposits	79,000
Equity capital	11,000
Intangibles assets, net	2,200
Goodwill	$ 6,700
Fair value estimates period	1 year
Number of locations acquired | Location	2