Accounting for Certain Loans Acquired in a Transfer - Amounts of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 2,856,793	$ 2,831,995
Allowance for Loan Losses	(16,474)	(16,394)	$ (15,054)	$ (14,837)
Carrying Amount	2,834,885	2,809,308
Loans Purchased With Evidence of Credit Deterioration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	27,109	29,863
Allowance for Loan Losses	0	0
Carrying Amount	27,109	29,863
Loans Purchased With Evidence of Credit Deterioration [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	24,721	27,198
Loans Purchased With Evidence of Credit Deterioration [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,356	2,632
Loans Purchased With Evidence of Credit Deterioration [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	32	33
Loans Purchased With Evidence of Credit Deterioration [Member] | Heartland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	470	619
Allowance for Loan Losses	0	0
Carrying Amount	470	619
Loans Purchased With Evidence of Credit Deterioration [Member] | Heartland [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	265	390
Loans Purchased With Evidence of Credit Deterioration [Member] | Heartland [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	205	229
Loans Purchased With Evidence of Credit Deterioration [Member] | Summit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	4,116	4,523
Allowance for Loan Losses	0	0
Carrying Amount	4,116	4,523
Loans Purchased With Evidence of Credit Deterioration [Member] | Summit [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	3,289	3,653
Loans Purchased With Evidence of Credit Deterioration [Member] | Summit [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	827	870
Loans Purchased With Evidence of Credit Deterioration [Member] | Peoples Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	426	441
Allowance for Loan Losses	0	0
Carrying Amount	426	441
Loans Purchased With Evidence of Credit Deterioration [Member] | Peoples Bancorp Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	308	315
Loans Purchased With Evidence of Credit Deterioration [Member] | Peoples Bancorp Inc [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	118	126
Loans Purchased With Evidence of Credit Deterioration [Member] | Kosciusko Financial Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,038	1,241
Allowance for Loan Losses	0	0
Carrying Amount	1,038	1,241
Loans Purchased With Evidence of Credit Deterioration [Member] | Kosciusko Financial Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	823	838
Loans Purchased With Evidence of Credit Deterioration [Member] | Kosciusko Financial Inc [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	215	403
Loans Purchased With Evidence of Credit Deterioration [Member] | LaPorte Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,923	2,071
Allowance for Loan Losses	0	0
Carrying Amount	1,923	2,071
Loans Purchased With Evidence of Credit Deterioration [Member] | LaPorte Bancorp Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	900	1,034
Loans Purchased With Evidence of Credit Deterioration [Member] | LaPorte Bancorp Inc [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	991	1,004
Loans Purchased With Evidence of Credit Deterioration [Member] | LaPorte Bancorp Inc [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	32	33
Loans Purchased With Evidence of Credit Deterioration [Member] | Lafayette Community Bancorp [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	3,655	4,271
Allowance for Loan Losses	0	0
Carrying Amount	3,655	4,271
Loans Purchased With Evidence of Credit Deterioration [Member] | Lafayette Community Bancorp [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	3,655	4,271
Loans Purchased With Evidence of Credit Deterioration [Member] | Wolverine Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	15,481	16,697
Allowance for Loan Losses	0	0
Carrying Amount	15,481	16,697
Loans Purchased With Evidence of Credit Deterioration [Member] | Wolverine Bancorp Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 15,481	$ 16,697