Recent Developments Including Mergers and Acquisitions - Additional Information (Detail) $ in Thousands				9 Months Ended
	Aug. 31, 2018 USD ($) ft² Floor	Jan. 11, 2018 USD ($) Location	Jul. 31, 2017 shares	Sep. 30, 2018 USD ($) Community	Dec. 31, 2017 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Goodwill				$ 79,733	$ 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		$ 6,600
Fair value estimates period		1 year
Number of locations acquired | Location		2
BFTower LLC [Member] | Cotter Ranch Tower [Member]
Business Combination Separately Recognized Transactions [Line Items]
Contract price	$ 21,000
Number of square feet | ft²	507,000
Number of floors | Floor	36
Date of acquisition	Aug. 31, 2018