Accounting for Certain Loans Acquired in a Transfer - Accretable Yield or Income Expected to be Collected (Detail) - Loans Purchased With Evidence of Credit Deterioration [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 5,165	$ 3,457
Additions	0	0
Accretion	(642)	(433)
Reclassification from nonaccretable difference	0	0
Disposals	(53)	(119)
Ending balance	4,470	2,905
Heartland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	452	557
Additions	0	0
Accretion	(59)	(34)
Reclassification from nonaccretable difference	0	0
Disposals		(6)
Ending balance	393	517
Summit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	147	502
Additions	0	0
Accretion	(18)	(93)
Reclassification from nonaccretable difference	0	0
Disposals	(2)	(2)
Ending balance	127	407
Peoples Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance		389
Additions	0	0
Accretion		(194)
Reclassification from nonaccretable difference	0	0
Disposals		(1)
Ending balance		194
Kosciusko Financial Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	386	530
Additions	0	0
Accretion	(20)	(31)
Reclassification from nonaccretable difference	0	0
Ending balance	366	499
LaPorte Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	980	1,479
Additions	0	0
Accretion	(40)	(81)
Reclassification from nonaccretable difference	0	0
Disposals	(7)	(110)
Ending balance	933	$ 1,288
Lafayette Community Bancorp [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	933
Additions	0
Accretion	(118)
Reclassification from nonaccretable difference	0
Disposals	(2)
Ending balance	813
Wolverine Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,267
Additions	0
Accretion	(387)
Reclassification from nonaccretable difference	0
Disposals	(42)
Ending balance	$ 1,838