Acquisitions - Schedule of Final Purchase Price of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Oct. 17, 2017
Loans
Goodwill	$ 119,880	$ 119,880
Wolverine Bancorp Inc [Member]
Assets
Cash and due from banks			$ 44,450
Loans
Total loans			310,667
Premises and equipment, net			2,941
FRB and FHLB stock			2,700
Goodwill			26,827
Core deposit intangible			2,024
Interest receivable			584
Other assets			3,897
Total assets purchased			394,090
Common shares issued			62,111
Cash paid			31,662
Total estimated purchase price			93,773
Deposits
Non-interest bearing			25,221
NOW accounts			8,026
Savings and money market			129,044
Certificates of deposit			94,688
Total deposits			256,979
Borrowings			36,970
Interest payable			214
Other liabilities			6,154
Total liabilities assumed			300,317
Wolverine Bancorp Inc [Member] | Residential Mortgage [Member]
Loans
Total loans			30,603
Wolverine Bancorp Inc [Member] | Commercial [Member]
Loans
Total loans			276,167
Wolverine Bancorp Inc [Member] | Consumer [Member]
Loans
Total loans			$ 3,897