Acquisitions - Additional Information (Detail) - USD ($)							9 Months Ended
		Oct. 17, 2017	Sep. 01, 2017	Feb. 03, 2017	Jul. 18, 2016	Jun. 01, 2016	Sep. 30, 2017	Sep. 30, 2016	Aug. 31, 2017	Dec. 31, 2016	Nov. 07, 2016	Aug. 31, 2016
Business Acquisition [Line Items]
Acquisition of goodwill							$ 93,750,000			$ 76,941,000
Common stock, shares outstanding	[1]						23,325,459			22,171,596
Kosciusko Financial Inc [Member]
Business Acquisition [Line Items]
Exchange ratio per share						451.83%
Cash paid for each share						$ 81.75
Business combination stock transferred, percentage of consideration						65.00%
Business combination cash transferred, percentage of consideration						35.00%
Common stock issued						873,430
Market closing price per share						$ 16.57
Estimated transaction value						$ 23,000,000
Costs related to the acquisition						1,600,000
Acquisition of goodwill						6,443,000		$ 6,400,000				$ 6,900,000
Net intangible assets acquired						526,000
Purchase price of the business assets, portion deductible						0
Core deposit intangible amortization period							10 years
Total estimated purchase price						22,983,000
Payments received as cash consideration								(30,437,000)
Reported total assets of acquiree						155,873,000
Total deposits held by acquiree						$ 132,890,000
Kosciusko Financial Inc [Member] | Maximum [Member]
Business Acquisition [Line Items]
Number of shares owned						100
Share of common stock outstanding per share						$ 81.75
LaPorte Bancorp Inc [Member]
Business Acquisition [Line Items]
Exchange ratio per share					94.35%
Cash paid for each share					$ 17.50
Business combination stock transferred, percentage of consideration					65.00%
Business combination cash transferred, percentage of consideration					35.00%
Common stock issued					3,421,488
Market closing price per share					$ 18.36
Estimated transaction value					$ 98,600,000
Costs related to the acquisition					4,000,000
Acquisition of goodwill					20,993,000
Net intangible assets acquired					2,514,000
Purchase price of the business assets, portion deductible					0
Core deposit intangible amortization period							10 years
Total estimated purchase price					98,634,000
Goodwill adjustment							$ 703,000
Payments received as cash consideration								$ (116,521,000)
Reported total assets of acquiree					549,537,000
Total deposits held by acquiree					$ 450,903,000
Central National Bank & Trust [Member]
Business Acquisition [Line Items]
Acquisition of goodwill											$ 609,000
Net intangible assets acquired											190,000
Purchase price of the business assets, portion deductible											0
Core deposit intangible amortization period							10 years
Total estimated purchase price											5,311,000
Performing portion of the loan portfolio acquired											10,800,000
Estimated fair value of performing portion of the loan portfolio											10,500,000
Reported total assets of acquiree											56,219,000
Total deposits held by acquiree											$ 50,908,000
First Farmers Bank & Trust Co [Member]
Business Acquisition [Line Items]
Acquisition of goodwill				$ 0
Net intangible assets acquired				$ 463,000
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
Lafayette Community Bancorp [Member]
Business Acquisition [Line Items]
Exchange ratio per share			58.78%
Cash paid for each share			$ 1.73
Common stock issued			1,091,259
Market closing price per share									$ 26.17
Estimated transaction value									$ 34,500,000
Costs related to the acquisition			$ 1,500,000
Acquisition of goodwill			16,106,000
Net intangible assets acquired			777,000
Purchase price of the business assets, portion deductible			0
Core deposit intangible amortization period							10 years
Total estimated purchase price			$ 34,529,000
Payments received as cash consideration							$ (20,425,000)
Common stock, shares outstanding			1,856,679
Ownership interest percentage			5.00%
Loan portfolio acquired			$ 134,400,000
Discount on loan portfolio acquired			3,400,000
Reported total assets of acquiree			186,659,000
Total deposits held by acquiree			$ 152,130,000
Lafayette Community Bancorp [Member] | Maximum [Member]
Business Acquisition [Line Items]
Number of shares owned			100
Share of common stock outstanding per share			$ 17.25
Horizon [Member]
Business Acquisition [Line Items]
Reported total assets of acquiree							3,900,000,000
Total deposits held by acquiree							2,900,000,000
Total net loans of acquiree							$ 2,700,000,000
Wolverine Bancorp Inc [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Exchange ratio per share		101.52%
Cash paid for each share		$ 14.00
Common stock issued		2,161,610
Market closing price per share		$ 29.06
Estimated transaction value		$ 95,100,000
Common stock, shares outstanding		2,129,331
Reported total assets of acquiree		$ 363,200,000
Total deposits held by acquiree		256,500,000
Total net loans of acquiree		$ 308,100,000

[1] Adjusted for 3:2 stock split on November 14, 2016.