Loans and Allowance for Loan Losses (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) alternative	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 947,000	$ 1,055,000	$ 8,667,000
Gross loans				$ 2,607,624,000	$ 2,658,136,000
Loans Receivable Held-for-sale, Amount				1,233,000	1,508,000
loans and leases receivable gross excluding loans held for sale				2,606,391,000	2,656,628,000
Less [Abstract]
Net deferred loan fees, premiums and discounts				10,630,000	13,617,000
Allowance for loan losses	26,189,000	19,977,000	19,977,000	26,704,000	26,189,000	$ 20,771,000
Net loans				2,569,057,000	2,616,822,000
Increase (Decrease) in loans receivable	(47,800,000)
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	$ 26,189,000	19,977,000	19,977,000
Provision for Loan and Lease Losses	947,000	1,055,000
Losses charged off	(568,000)	(476,000)	(2,993,000)
Recoveries	136,000	215,000	538,000
Balance, end of period	$ 26,704,000	20,771,000	26,189,000
Ending Balance [Abstract]
Individually evaluated for impairment				2,515,000	2,044,000	511,000
Collectively evaluated for impairment				23,484,000	22,910,000	20,255,000
Ending Balance [Abstract]
Individually evaluated for impairment				14,282,000	16,980,000	13,079,000
Collectively evaluated for impairment				2,568,096,000	2,611,916,000	1,964,365,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				2,596,994,000	2,644,519,000	1,977,697,000
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				50,234,000	51,013,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				49,179,000	50,619,000
Agricultural real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				237,437,000	232,409,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				236,864,000	231,700,000
1-4 Family residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				363,569,000	374,751,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				362,617,000	373,518,000
Period When Loans are Charged-down	180 days
Multifamily residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				177,949,000	186,393,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				175,903,000	184,051,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				909,176,000	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				905,679,000	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				1,738,365,000	1,756,222,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				1,730,242,000	1,746,738,000
Agricultural loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				118,125,000	136,125,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				118,026,000	135,877,000
Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				551,837,000	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				550,853,000	557,011,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	$ 243,000	103,000	602,000
Gross loans				87,503,000	92,744,000
Less [Abstract]
Allowance for loan losses	932,000	803,000	803,000	1,004,000	932,000	861,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	932,000	803,000	803,000
Losses charged off	(271,000)	(136,000)	(787,000)
Recoveries	100,000	91,000	314,000
Balance, end of period	1,004,000	861,000	932,000
Ending Balance [Abstract]
Individually evaluated for impairment				3,000	3,000	0
Collectively evaluated for impairment				1,001,000	929,000	861,000
Ending Balance [Abstract]
Individually evaluated for impairment				171,000	166,000	170,000
Collectively evaluated for impairment				93,948,000	99,872,000	39,650,000
Financing Receivable, Net				94,119,000	100,041,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				86,540,000	91,516,000	39,820,000
All other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				111,794,000	113,925,000
Ending Balance [Abstract]
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				111,333,000	113,377,000
Agricultural and Farm Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				355,600,000	368,500,000
Less [Abstract]
Increase (Decrease) in loans receivable	$ (12,900,000)
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				267,100,000	276,100,000
Less [Abstract]
Increase (Decrease) in loans receivable	$ (9,000,000)
Motels and Hotels Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				128,000,000
Non-residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				228,300,000
Residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				289,200,000
Other Gambling Industries [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans				103,000,000
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)	$ 584,000	936,000	6,070,000
Less [Abstract]
Allowance for loan losses	21,556,000	16,546,000	16,546,000	21,947,000	21,556,000	17,368,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	21,556,000	16,546,000	16,546,000
Losses charged off	(215,000)	(237,000)	(1,227,000)
Recoveries	22,000	123,000	167,000
Balance, end of period	21,947,000	17,368,000	21,556,000
Ending Balance [Abstract]
Individually evaluated for impairment				2,257,000	1,816,000	486,000
Collectively evaluated for impairment				18,994,000	18,514,000	16,877,000
Ending Balance [Abstract]
Individually evaluated for impairment				10,303,000	14,422,000	11,592,000
Collectively evaluated for impairment				1,747,455,000	1,756,908,000	1,417,379,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				1,770,557,000	1,784,741,000	1,429,224,000
Agricultural/ Agricultural Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	236,000	(161,000)	548,000
Less [Abstract]
Allowance for loan losses	2,197,000	1,742,000	1,742,000	2,412,000	2,197,000	1,581,000
Allowance for Loan Losses [Abstract]
Balance, beginning of period	2,197,000	1,742,000	1,742,000
Losses charged off	(30,000)	0	(93,000)
Recoveries	9,000	0	0
Balance, end of period	2,412,000	1,581,000	2,197,000
Ending Balance [Abstract]
Individually evaluated for impairment				24,000	0	5,000
Collectively evaluated for impairment				2,388,000	2,197,000	1,576,000
Ending Balance [Abstract]
Individually evaluated for impairment				45,000	32,000	202,000
Collectively evaluated for impairment				354,432,000	367,175,000	196,173,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				354,477,000	367,211,000	196,375,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	(116,000)	177,000	1,447,000
Less [Abstract]
Allowance for loan losses	$ 1,504,000	886,000	886,000	1,341,000	1,504,000	961,000
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	$ 1,504,000	886,000	886,000
Losses charged off	(52,000)	(103,000)	(886,000)
Recoveries	5,000	1,000	57,000
Balance, end of period	1,341,000	961,000	1,504,000
Ending Balance [Abstract]
Individually evaluated for impairment				231,000	225,000	20,000
Collectively evaluated for impairment				1,101,000	1,270,000	941,000
Ending Balance [Abstract]
Individually evaluated for impairment				3,763,000	2,360,000	1,115,000
Collectively evaluated for impairment				372,261,000	387,961,000	311,163,000
Loans and Leases Receivable, Including Held for Sale, Net of Deferred Income				377,841,000	392,526,000	312,278,000
Unallocated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)	0	0	0
Less [Abstract]
Allowance for loan losses	0	0	0	0	0	0
Allowance for Loan Losses [Abstract]
Balance, beginning of period	0	0	0
Losses charged off	0	0	0
Recoveries	0	0	0
Balance, end of period	$ 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				13,911,000	14,388,000
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				705,000	1,235,000	5,000
Ending Balance [Abstract]
Financing Receivable, Net				14,616,000	15,623,000	253,000
Financial Asset Acquired with Credit Deterioration [Member] | Construction and land development
Ending Balance [Abstract]
Financing Receivable, Net				278,000	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				1,817,000	2,206,000
Financial Asset Acquired with Credit Deterioration [Member] | Multifamily residential properties
Ending Balance [Abstract]
Financing Receivable, Net				3,864,000	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	0
Financial Asset Acquired with Credit Deterioration [Member] | Commercial/ Commercial Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				696,000	1,226,000	5,000
Ending Balance [Abstract]
Financing Receivable, Net				12,799,000	13,411,000	253,000
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	0
Financial Asset Acquired with Credit Deterioration [Member] | Residential Real Estate
Ending Balance [Abstract]
Financing Receivable, Allowance for Credit Losses				9,000	9,000	0
Ending Balance [Abstract]
Financing Receivable, Net				1,817,000	2,205,000	0
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