Fair Market Value of Financial Assets and Liabilities, Estimated Fair Value of Financial Instruments and Indication of Level Within Fair Value Hierarchy of Valuation Techniques (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial assets [Abstract]
Available-for-sale securities	$ 604,890	$ 585,761
Securities held-to-maturity	660	660
Mortgage servicing rights	3,706	3,484
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	194,468	175,274
Certificates of deposits in other banks	8,085	9,800
Available-for-sale securities	604,890	585,761
Securities held-to-maturity	659	659
Loans held for sale	1,145	1,033
Loans, net	3,083,052	3,086,789
Federal Home Loan Bank stock	17,927	17,927
Federal Reserve Bank stock	4,887	4,887
Accrued interest receivable	13,311	13,338
Mortgage servicing rights	3,706	3,484
Financial liabilities [Abstract]
Deposits	3,319,670	3,263,863
Repurchase agreements	244,822	243,814
Federal funds purchased	7,078	7,312
Advances from Federal Home Loan Bank	822	845
Long-term debt	59,341	59,341
Accrued interest payable	2,927	2,228
Unrecognized financial instruments [Abstract]
Letters of credit	0	0
Commitments to extend credit	0	0
Forward sale commitments	0	0
Fair Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets [Abstract]
Cash and cash equivalents	194,468	175,274
Certificates of deposits in other banks	0	0
Available-for-sale securities	86,316	88,956
Securities held-to-maturity	0	0
Loans held for sale	1,177	1,060
Loans, net	0	0
Federal Home Loan Bank stock	0	0
Federal Reserve Bank stock	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Financial liabilities [Abstract]
Deposits	825,345	790,930
Repurchase agreements	0	0
Federal funds purchased	0	0
Advances from Federal Home Loan Bank	0	0
Long-term debt	0	0
Accrued interest payable	0	0
Unrecognized financial instruments [Abstract]
Letters of credit	0	0
Commitments to extend credit	0	0
Forward sale commitments	0	0
Fair Value [Member] | Significant Other Observable Inputs (Level 2) [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	0
Certificates of deposits in other banks	8,056	9,772
Available-for-sale securities	518,574	496,805
Securities held-to-maturity	660	660
Loans held for sale	0	0
Loans, net	0	0
Federal Home Loan Bank stock	17,927	17,927
Federal Reserve Bank stock	4,887	4,887
Accrued interest receivable	13,311	13,338
Mortgage servicing rights	0	0
Financial liabilities [Abstract]
Deposits	2,535,681	2,319,278
Repurchase agreements	0	0
Federal funds purchased	7,078	7,312
Advances from Federal Home Loan Bank	895	841
Long-term debt	0	0
Accrued interest payable	2,927	2,228
Unrecognized financial instruments [Abstract]
Letters of credit	0	0
Commitments to extend credit	0	0
Forward sale commitments	0	0
Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	0
Certificates of deposits in other banks	0	0
Available-for-sale securities	0	0
Securities held-to-maturity	0	0
Loans held for sale	0	0
Loans, net	3,154,449	3,092,437
Federal Home Loan Bank stock	0	0
Federal Reserve Bank stock	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	3,706	3,484
Financial liabilities [Abstract]
Deposits	0	0
Repurchase agreements	244,943	243,932
Federal funds purchased	0	0
Advances from Federal Home Loan Bank	0	0
Long-term debt	44,166	44,166
Accrued interest payable	0	0
Unrecognized financial instruments [Abstract]
Letters of credit	0	0
Commitments to extend credit	0	0
Forward sale commitments	$ 0	$ 0