Accounting for Certain Loans Acquired in a Transfer - Amounts of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 1,720,072	$ 1,749,131
Carrying amount, net of allowance	1,702,424	1,730,806
Loans Purchased With Evidence Of Credit Deterioration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	9,912	10,390
Carrying amount, net of allowance	9,912	10,327
Loans Purchased With Evidence Of Credit Deterioration [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	7,738	8,261
Loans Purchased With Evidence Of Credit Deterioration [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,164	2,088
Loans Purchased With Evidence Of Credit Deterioration [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	10	41
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,959	2,332
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,282	1,633
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	676	693
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1	6
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	6,702	6,818
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	5,494	5,567
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,199	1,216
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	9	35
Loans Purchased With Evidence Of Credit Deterioration [Member] | Peoples Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,251	1,240
Loans Purchased With Evidence Of Credit Deterioration [Member] | Peoples Bancorp Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	962	1,061
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	$ 289	$ 179