Loans and Allowance for Loan Losses (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) alternative	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 91,000	$ 1,877,000	$ 1,038,000	$ 2,932,000	$ 8,667,000
Gross loans						$ 2,554,713,000	$ 2,658,136,000
Loans Receivable Held-for-sale, Amount						1,717,000	1,508,000
loans and leases receivable gross excluding loans held for sale						2,552,996,000	2,656,628,000
Less [Abstract]
Net deferred loan fees, premiums and discounts						8,170,000	13,617,000
Allowance for loan losses	26,704,000	20,771,000	26,189,000	19,977,000	19,977,000	26,359,000	26,189,000	$ 22,045,000
Net loans						2,518,467,000	2,616,822,000
Increase (Decrease) in loans receivable			(98,400,000)
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	26,704,000	20,771,000	$ 26,189,000	19,977,000	19,977,000
Provision for Loan and Lease Losses	91,000	1,877,000	1,038,000	2,932,000
Losses charged off	(542,000)	(719,000)	(1,110,000)	(1,195,000)	(2,993,000)
Recoveries	106,000	116,000	242,000	331,000	538,000
Balance, end of period	26,359,000	22,045,000	$ 26,359,000	22,045,000	26,189,000
Ending Balance [Abstract]
Individually evaluated for impairment						1,487,000	2,044,000	542,000
Collectively evaluated for impairment						24,082,000	22,910,000	21,503,000
Financing Receivable, Allowance for Credit Losses								0
Ending Balance [Abstract]
Individually evaluated for impairment						14,708,000	16,980,000	15,209,000
Collectively evaluated for impairment						2,522,845,000	2,611,916,000	2,345,448,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						2,546,543,000	2,644,519,000	2,376,683,000
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						57,410,000	51,013,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						57,069,000	50,619,000
Agricultural real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						230,421,000	232,409,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						229,924,000	231,700,000
1-4 Family residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						355,922,000	374,751,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						355,143,000	373,518,000
Period When Loans are Charged-down			180 days
Multifamily residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						169,370,000	186,393,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						167,709,000	184,051,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						892,269,000	911,656,000
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						888,711,000	906,850,000
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						1,705,392,000	1,756,222,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						1,698,556,000	1,746,738,000
Agricultural loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						118,150,000	136,125,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						118,216,000	135,877,000
Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						530,749,000	559,120,000
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						530,405,000	557,011,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	218,000	132,000	$ 461,000	235,000	602,000
Gross loans						85,630,000	92,744,000
Less [Abstract]
Allowance for loan losses	1,004,000	861,000	932,000	803,000	803,000	1,090,000	932,000	928,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	1,004,000	861,000	932,000	803,000	803,000
Losses charged off	(217,000)	(128,000)	(488,000)	(264,000)	(787,000)
Recoveries	85,000	63,000	185,000	154,000	314,000
Balance, end of period	1,090,000	928,000	1,090,000	928,000	932,000
Ending Balance [Abstract]
Individually evaluated for impairment						2,000	3,000	0
Collectively evaluated for impairment						1,088,000	929,000	928,000
Ending Balance [Abstract]
Individually evaluated for impairment						166,000	166,000	153,000
Collectively evaluated for impairment						92,326,000	99,872,000	73,773,000
Financing Receivable, Net						92,492,000	100,041,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						84,907,000	91,516,000	73,932,000
All other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						114,792,000	113,925,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						114,459,000	113,377,000
Agricultural and Farm Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						348,600,000	368,500,000
Less [Abstract]
Increase (Decrease) in loans receivable			$ (19,900,000)
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						264,100,000	276,100,000
Less [Abstract]
Increase (Decrease) in loans receivable			$ (12,000,000)
Motels and Hotels Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						127,100,000
Non-residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						235,000,000
Residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						288,400,000
Other Gambling Industries [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans						90,100,000
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)	(1,042,000)	1,334,000	$ (458,000)	2,270,000	6,070,000
Less [Abstract]
Allowance for loan losses	21,947,000	17,368,000	21,556,000	16,546,000	16,546,000	20,663,000	21,556,000	18,166,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	21,947,000	17,368,000	21,556,000	16,546,000	16,546,000
Losses charged off	(258,000)	(536,000)	(473,000)	(773,000)	(1,227,000)
Recoveries	16,000	0	38,000	123,000	167,000
Balance, end of period	20,663,000	18,166,000	20,663,000	18,166,000	21,556,000
Ending Balance [Abstract]
Individually evaluated for impairment						981,000	1,816,000	493,000
Collectively evaluated for impairment						18,899,000	18,514,000	17,673,000
Ending Balance [Abstract]
Individually evaluated for impairment						11,253,000	14,422,000	12,163,000
Collectively evaluated for impairment						1,717,655,000	1,756,908,000	1,593,116,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						1,736,094,000	1,784,741,000	1,617,579,000
Agricultural/ Agricultural Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	545,000	319,000	781,000	158,000	548,000
Less [Abstract]
Allowance for loan losses	2,412,000	1,581,000	2,197,000	1,742,000	1,742,000	2,957,000	2,197,000	1,900,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	2,412,000	1,581,000	2,197,000	1,742,000	1,742,000
Losses charged off	0	0	(30,000)	0	(93,000)
Recoveries	0	0	9,000	0	0
Balance, end of period	2,957,000	1,900,000	2,957,000	1,900,000	2,197,000
Ending Balance [Abstract]
Individually evaluated for impairment						0	0	30,000
Collectively evaluated for impairment						2,957,000	2,197,000	1,870,000
Ending Balance [Abstract]
Individually evaluated for impairment						103,000	32,000	150,000
Collectively evaluated for impairment						347,611,000	367,175,000	294,768,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						347,714,000	367,211,000	295,156,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	370,000	92,000	254,000	269,000	1,447,000
Less [Abstract]
Allowance for loan losses	1,341,000	961,000	$ 1,504,000	886,000	886,000	1,649,000	1,504,000	1,051,000
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	1,341,000	961,000	$ 1,504,000	886,000	886,000
Losses charged off	(67,000)	(55,000)	(119,000)	(158,000)	(886,000)
Recoveries	5,000	53,000	10,000	54,000	57,000
Balance, end of period	1,649,000	1,051,000	1,649,000	1,051,000	1,504,000
Ending Balance [Abstract]
Individually evaluated for impairment						504,000	225,000	19,000
Collectively evaluated for impairment						1,138,000	1,270,000	1,032,000
Ending Balance [Abstract]
Individually evaluated for impairment						3,186,000	2,360,000	2,743,000
Collectively evaluated for impairment						365,253,000	387,961,000	383,791,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						370,243,000	392,526,000	390,016,000
Unallocated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	0	0	0	0	0
Less [Abstract]
Allowance for loan losses	0	0	0	0	0	0	0	0
Allowance for Loan Losses [Abstract]
Balance, beginning of period	0	0	0	0	0
Losses charged off	0	0	0	0	0
Recoveries	0	0	0	0	0
Balance, end of period	$ 0	$ 0	$ 0	$ 0	$ 0
Ending Balance [Abstract]
Individually evaluated for impairment						0	0	0
Collectively evaluated for impairment						0	0	0
Ending Balance [Abstract]
Individually evaluated for impairment						0	0	0
Collectively evaluated for impairment						0	0	0
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income						0	0	0
Financial Asset Acquired with Credit Deterioration [Member]
Less [Abstract]
Net loans						8,200,000	14,388,000
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						790,000	1,235,000	0
Ending Balance [Abstract]
Financing Receivable, Net						8,990,000	15,623,000	16,026,000
Financial Asset Acquired with Credit Deterioration [Member] | Construction and land development
Ending Balance [Abstract]
Financing Receivable, Net						270,000	2,558,000
Financial Asset Acquired with Credit Deterioration [Member] | Agricultural real estate
Ending Balance [Abstract]
Financing Receivable, Net						0	0
Financial Asset Acquired with Credit Deterioration [Member] | 1-4 Family residential properties
Ending Balance [Abstract]
Financing Receivable, Net						1,804,000	2,206,000
Financial Asset Acquired with Credit Deterioration [Member] | Multifamily residential properties
Ending Balance [Abstract]
Financing Receivable, Net						2,294,000	3,891,000
Financial Asset Acquired with Credit Deterioration [Member] | Agricultural loans
Ending Balance [Abstract]
Financing Receivable, Net						0	4,000
Financial Asset Acquired with Credit Deterioration [Member] | Commercial and industrial loans
Ending Balance [Abstract]
Financing Receivable, Net						0	15,000
Financial Asset Acquired with Credit Deterioration [Member] | Consumer loans
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						0	0	0
Ending Balance [Abstract]
Financing Receivable, Net						0	3,000	6,000
Financial Asset Acquired with Credit Deterioration [Member] | Commercial/ Commercial Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						783,000	1,226,000	0
Ending Balance [Abstract]
Financing Receivable, Net						7,186,000	13,411,000	12,300,000
Financial Asset Acquired with Credit Deterioration [Member] | Agricultural/ Agricultural Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						0	0	0
Ending Balance [Abstract]
Financing Receivable, Net						0	4,000	238,000
Financial Asset Acquired with Credit Deterioration [Member] | Residential Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						7,000	9,000	0
Ending Balance [Abstract]
Financing Receivable, Net						1,804,000	2,205,000	3,482,000
Financial Asset Acquired with Credit Deterioration [Member] | Unallocated
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses						0	0	0
Ending Balance [Abstract]
Financing Receivable, Net						$ 0	$ 0	$ 0