Business Combination and Asset Purchase - Additional Information (Detail)			3 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($) Office $ / shares shares	Dec. 13, 2013 USD ($)	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) shares
Business Acquisition [Line Items]
Common stock, shares outstanding | shares			12,230,000		10,400,000
Business combination, acquisition related costs			$ 1,300,000		$ 742,100
Business combination, acquisition related costs percentage					98.40%
Recognition of core deposit intangible asset with acquired purchase			$ 3,900,000
Remaining economic useful life			7 years
Goodwill			$ 47,340,000		$ 4,074,000
Fair value of assets acquired			257,200,000
Gross principal and contractual interest			359,200,000
Acquisition expected to be uncollectible			$ 4,700,000
Loan receivable weighted average life			70 months
Business combination, premises and equipment written down value			$ 1,200,000
Business combination, average remaining life			16 years 9 months
Fair value for certificate of deposit, valuation amount			$ 500,000
Fair value for certificate of deposit, amortization period			1 year 6 months
Fair value of Federal Home Loan Bank (FHLB) advances, valuation amount			$ 1,300,000
Fair value of Federal Home Loan Bank (FHLB) advances, amortization period			2 years 3 months 18 days
Carrying amount, allowance	$ 2,118,000		$ 2,081,000
Amortization expense			182,000	$ 42,000	42,000
Amortization expense			727,000
Core Deposits [Member] | Custar [Member]
Business Acquisition [Line Items]
Recognition of core deposit intangible asset with acquired purchase		$ 1,170,000
Limberlost Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Common stock issued in exchange of each outstanding share | shares	1,830,000
Total consideration for acquisition	$ 78,902,000		78,902,000
Cash paid for acquisition			8,465,000
Stock issued for acquisition			70,437,000
Goodwill	43,266,000
Limberlost Bancshares, Inc. [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Recognition of core deposit intangible asset with acquired purchase	$ 3,900,000
Remaining economic useful life	7 years
Consulting Fees [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs			18,300		340,000
Other General and Administrative Expenses [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs			238,300		331,500
Data Processing [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs			891,700		$ 58,600
Employee Benefits [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs			107,600
ATM Expense [Member]
Business Acquisition [Line Items]
Business combination, acquisition related costs			31,400
Bank of Geneva [Member]
Business Acquisition [Line Items]
Number of full-service offices | Office	6
Pro-forma operating revenue, net of tax			2,100,000
Limberlost Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Common stock issued in exchange of each outstanding share | shares	1,830
Cash payment for exchange of each share | $ / shares	$ 8,465.00
Common stock, shares outstanding | shares	1,000
Share price | $ / shares	$ 38.49
Total consideration for acquisition	$ 78,900,000
Cash paid for acquisition	8,500,000		$ 8,465,000
Stock issued for acquisition	$ 70,400,000