Acquisitions - Additional Information (Detail) - USD ($)				3 Months Ended
	Oct. 17, 2017	Sep. 01, 2017	Feb. 03, 2017	Mar. 31, 2018	Dec. 31, 2017	Aug. 31, 2017
Business Acquisition [Line Items]
Common stock, shares outstanding				25,555,235	25,529,819
Acquisition of goodwill				$ 119,880,000	$ 119,880,000
Ownership interest percentage		5.00%
Wolverine Bancorp Inc [Member]
Business Acquisition [Line Items]
Exchange ratio per share	101.52%
Cash paid for each share	$ 14.00
Common stock, shares outstanding	2,129,331
Common stock issued	2,160,697
Market closing price per share	$ 29.06
Estimated transaction value	$ 93,800,000
Costs related to the acquisition	1,900,000
Total estimated purchase price	93,773,000
Net intangible assets acquired	2,024,000
Acquisition of goodwill	26,827,000
Purchase price of the business assets, portion deductible	$ 0
Core deposit intangible amortization period				10 years
Lafayette Community Bancorp [Member]
Business Acquisition [Line Items]
Exchange ratio per share		58.78%
Cash paid for each share		$ 1.73
Common stock, shares outstanding		1,856,679
Common stock issued		1,091,259
Market closing price per share						$ 26.17
Estimated transaction value						$ 34,500,000
Costs related to the acquisition		$ 1,700,000
Total estimated purchase price		34,465,000
Net intangible assets acquired		2,085,000
Acquisition of goodwill		15,408,000
Purchase price of the business assets, portion deductible		$ 0
Core deposit intangible amortization period				10 years
Gain on remeasurement of equity interest					$ 530,000
Lafayette Community Bancorp [Member] | Maximum [Member]
Business Acquisition [Line Items]
Number of shares owned		100
Share of common stock outstanding per share		$ 17.25
First Farmers Bank & Trust Co [Member]
Business Acquisition [Line Items]
Net intangible assets acquired			$ 452,000,000
Acquisition of goodwill			$ 0
Core deposit intangible amortization period			10 years
Payments received as cash consideration			$ 11,000,000
Business acquisition, loans assumed			$ 3,400,000
Premium on deposits paid			3.00%
Business acquisition, customer deposit balances			$ 14,800,000