Loans and Allowance for Loan Losses (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) alternative	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 1,055,000	$ 1,722,000	$ 7,462,000
Gross loans				$ 1,983,555,000	$ 1,946,114,000
Loans Receivable Held-for-sale, Amount				1,309,000	1,025,000
loans and leases receivable gross excluding loans held for sale				1,982,246,000	1,945,089,000
Less [Abstract]
Net deferred loan fees, premiums and discounts				5,858,000	6,613,000
Allowance for loan losses	19,977,000	16,753,000	16,753,000	20,771,000	19,977,000	$ 17,846,000
Net loans				1,955,617,000	1,918,499,000
Increase (Decrease) in loans receivable	37,100,000
Impaired Loans [Abstract]
Minimum value of loans individually measured for impairment	$ 250,000
Number of alternatives for measuring impaired loans receivable | alternative	3
Allowance for Loan Losses [Abstract]
Balance, beginning of period	$ 19,977,000	16,753,000	16,753,000
Provision for Loan and Lease Losses	1,055,000	1,722,000
Losses charged off	(476,000)	(763,000)	(5,195,000)
Recoveries	215,000	134,000	957,000
Balance, end of period	$ 20,771,000	17,846,000	19,977,000
Ending Balance [Abstract]
Individually evaluated for impairment				511,000	614,000	1,070,000
Collectively evaluated for impairment				20,255,000	19,354,000	16,751,000
Ending Balance [Abstract]
Individually evaluated for impairment				13,079,000	13,086,000	16,223,000
Collectively evaluated for impairment				1,964,365,000	1,926,156,000	1,771,471,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				1,977,697,000	1,939,501,000	1,795,662,000
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				109,188,000	107,721,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				109,076,000	107,594,000
Agricultural real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				122,598,000	127,232,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				122,564,000	127,183,000
1-4 Family residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				290,567,000	294,483,000
Ending Balance [Abstract]
Period When Loans are Charged-down	180 days
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				289,899,000	293,667,000
Multifamily residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				61,049,000	61,966,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				60,881,000	61,798,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				702,023,000	684,639,000
Loans Receivable Additional Information [Abstract]
Debt coverage ratio	1.20x
Amortization period of loans	twenty years
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				699,142,000	681,757,000
Commercial real estate | Minimum [Member]
Loans Receivable Additional Information [Abstract]
Maximum Loan-to-value Ratio (in hundredths)	65.00%
Commercial real estate | Maximum [Member]
Loans Receivable Additional Information [Abstract]
Maximum Loan-to-value Ratio (in hundredths)	80.00%
Loans secured by real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				1,285,425,000	1,276,041,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				1,281,562,000	1,271,999,000
Agricultural loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				74,282,000	86,602,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				74,342,000	86,631,000
Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				459,259,000	445,378,000
Loans Receivable Additional Information [Abstract]
Maximum Loan-to-value Ratio (in hundredths)	80.00%
Amortization period of loans	seven years
Loans Receivable, Time Period	one year
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				458,691,000	444,263,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 103,000	50,000	361,000
Gross loans				29,107,000	30,070,000
Less [Abstract]
Allowance for loan losses	803,000	693,000	693,000	861,000	803,000	751,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	803,000	693,000	693,000
Losses charged off	(136,000)	(102,000)	(521,000)
Recoveries	91,000	110,000	270,000
Balance, end of period	861,000	751,000	803,000
Ending Balance [Abstract]
Individually evaluated for impairment				0	1,000	1,000
Collectively evaluated for impairment				861,000	802,000	750,000
Ending Balance [Abstract]
Individually evaluated for impairment				170,000	200,000	301,000
Collectively evaluated for impairment				39,650,000	38,870,000	38,451,000
Loans Receivable, Net				39,820,000	39,070,000	38,752,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				28,784,000	29,749,000
All other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				135,482,000	108,023,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				134,318,000	106,859,000
Agricultural and Farm Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				196,900,000	213,800,000
Less [Abstract]
Increase (Decrease) in loans receivable	$ (16,900,000)
Loans Receivable Additional Information [Abstract]
Maximum Loan-to-value Ratio (in hundredths)	65.00%
Amortization period of loans	twenty five years
Loans Receivable, Time Period	one year
Other Grain Farming [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				158,300,000	170,800,000
Less [Abstract]
Increase (Decrease) in loans receivable	$ (12,500,000)
Motels and Hotels Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				118,400,000
Non-residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				187,400,000
Residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				131,300,000
Other Gambling Industries [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				97,800,000
Unsecured Open-end Loans [Member]
Ending Balance [Abstract]
Period When Loans are Charged-down	180 days
Other Secured Loans [Member]
Ending Balance [Abstract]
Period When Loans are Charged-down	120 days
Commercial/ Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 936,000	1,466,000	6,884,000
Less [Abstract]
Allowance for loan losses	16,546,000	12,901,000	12,901,000	17,368,000	16,546,000	13,771,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	16,546,000	12,901,000	12,901,000
Losses charged off	(237,000)	(612,000)	(3,795,000)
Recoveries	123,000	16,000	556,000
Balance, end of period	17,368,000	13,771,000	16,546,000
Ending Balance [Abstract]
Individually evaluated for impairment				486,000	586,000	316,000
Collectively evaluated for impairment				16,877,000	15,951,000	13,455,000
Ending Balance [Abstract]
Individually evaluated for impairment				11,592,000	11,372,000	10,656,000
Collectively evaluated for impairment				1,417,379,000	1,360,156,000	1,190,272,000
Loans Receivable, Net				1,429,224,000	1,371,787,000	1,204,748,000
Agricultural/ Agricultural Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	(161,000)	69,000	153,000
Less [Abstract]
Allowance for loan losses	1,742,000	2,249,000	2,249,000	1,581,000	1,742,000	2,319,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	1,742,000	2,249,000	2,249,000
Losses charged off	0	0	(662,000)
Recoveries	0	1,000	2,000
Balance, end of period	1,581,000	2,319,000	1,742,000
Ending Balance [Abstract]
Individually evaluated for impairment				5,000	2,000	709,000
Collectively evaluated for impairment				1,576,000	1,740,000	1,610,000
Ending Balance [Abstract]
Individually evaluated for impairment				202,000	488,000	1,173,000
Collectively evaluated for impairment				196,173,000	213,033,000	198,588,000
Loans Receivable, Net				196,375,000	213,521,000	199,761,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	177,000	146,000	100,000
Less [Abstract]
Allowance for loan losses	$ 886,000	874,000	874,000	961,000	886,000	978,000
Loans Receivable Additional Information [Abstract]
Maximum Loan-to-value Ratio (in hundredths)	80.00%
Amortization period of loans	twenty five years
Balloon period	five years
Allowance for Loan Losses [Abstract]
Balance, beginning of period	$ 886,000	874,000	874,000
Losses charged off	(103,000)	(49,000)	(217,000)
Recoveries	1,000	7,000	129,000
Balance, end of period	961,000	978,000	886,000
Ending Balance [Abstract]
Individually evaluated for impairment				20,000	25,000	44,000
Collectively evaluated for impairment				941,000	861,000	909,000
Ending Balance [Abstract]
Individually evaluated for impairment				1,115,000	1,026,000	4,093,000
Collectively evaluated for impairment				311,163,000	314,097,000	344,160,000
Loans Receivable, Net				312,278,000	315,123,000	352,401,000
Unallocated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	0	(9,000)	(36,000)
Less [Abstract]
Allowance for loan losses	0	36,000	36,000	0	0	27,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	0	36,000	36,000
Losses charged off	0	0	0
Recoveries	0	0	0
Balance, end of period	$ 0	$ 27,000	$ 0
Ending Balance [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				0	0	27,000
Ending Balance [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				0	0	0
Loans Receivable, Net				0	0	0
Receivables Acquired with Deteriorated Credit Quality [Member]
Less [Abstract]
Net loans				248,000	250,000
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				5,000	9,000	25,000
Ending Balance [Abstract]
Financing Receivable, Net				253,000	259,000	7,968,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial and industrial loans
Ending Balance [Abstract]
Financing Receivable, Net				4,000	8,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer loans
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				0	0	0
Ending Balance [Abstract]
Financing Receivable, Net				0	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial/ Commercial Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				5,000	9,000	0
Ending Balance [Abstract]
Financing Receivable, Net				253,000	259,000	3,820,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Agricultural/ Agricultural Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				0	0	0
Ending Balance [Abstract]
Financing Receivable, Net				0	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Residential Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				0	0	25,000
Ending Balance [Abstract]
Financing Receivable, Net				0	0	4,148,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Unallocated
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				0	0	0
Ending Balance [Abstract]
Financing Receivable, Net				$ 0	$ 0	$ 0