Loans and Allowance for Loan Losses, Part II (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Loans Receivable Aging Analysis [Abstract]
Total Past Due	$ 14,744,000		$ 10,919,000
Current	1,962,953,000		1,928,582,000
Total Loans Receivable	1,977,697,000	$ 1,795,662,000	1,939,501,000
Total Loans 90 Days & Accruing	$ 0		0
Number of days past due when interest is not accrued	ninety days
Period of satisfactory performance before returning to accrual status	not less than six months
Loans with a specific allowance [Abstract]
Recorded Balance	$ 13,332,000		13,345,000
Unpaid Principal Balance	14,977,000		14,747,000
Specific Allowance	516,000		623,000
Loans without a specific allowance [Abstract]
Recorded Balance	4,537,000		4,168,000
Unpaid Principal Balance	4,208,000		4,470,000
Total Loans [Abstract]
Recorded Balance	17,869,000		17,513,000
Unpaid Principal Balance	19,185,000		19,217,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	18,473,000	25,750,000
Interest income recognized	13,000	56,000
Loans Receivable, Modifications, Still Accruing Interest	32,000		37,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	16,870,000		16,659,000
Interest Lost on Nonaccrual Loans	608,000	$ 287,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	8,238,000		8,898,000
Financing Receivable Modifications Performing Recorded Investment	$ 999,000		854,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	1
Recorded Balance of Troubled Debt Restructurings [Abstract]
Financing Receivables, Modifications during Period, Number	1	0
Financing Receivables, Modifications during Period, Balance	$ 161,000	$ 0
Subsequent Default, Number of Days Past Due	90 days
Real Estate Acquired Through Foreclosure	$ 1,951,000		2,754,000
Mortgage Loans Secured By Real Estate In Foreclosure	938,000		404,000
Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	1,908,328,000		1,860,539,000
Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	33,157,000		42,675,000
Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	36,212,000		36,287,000
Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Construction and land development
Loans Receivable Aging Analysis [Abstract]
Total Past Due	503,000		74,000
Current	108,573,000		107,520,000
Total Loans Receivable	109,076,000		107,594,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	0		0
Unpaid Principal Balance	0		0
Specific Allowance	0		0
Loans without a specific allowance [Abstract]
Recorded Balance	45,000		0
Unpaid Principal Balance	45,000		0
Total Loans [Abstract]
Recorded Balance	45,000		0
Unpaid Principal Balance	45,000		0
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	46,000	0
Interest income recognized	0	0
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	45,000		0
Construction and land development | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	108,582,000		107,140,000
Construction and land development | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	449,000		454,000
Construction and land development | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	45,000		0
Construction and land development | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Agricultural real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,103,000		396,000
Current	121,461,000		126,787,000
Total Loans Receivable	122,564,000		127,183,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	0		276,000
Unpaid Principal Balance	0		276,000
Specific Allowance	0		0
Loans without a specific allowance [Abstract]
Recorded Balance	0		15,000
Unpaid Principal Balance	0		15,000
Total Loans [Abstract]
Recorded Balance	0		291,000
Unpaid Principal Balance	0		291,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	0	161,000
Interest income recognized	0	0
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	0		291,000
Agricultural real estate | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	117,516,000		120,767,000
Agricultural real estate | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	3,591,000		4,829,000
Agricultural real estate | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	1,457,000		1,587,000
Agricultural real estate | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
1-4 Family residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	4,014,000		5,328,000
Current	285,885,000		288,339,000
Total Loans Receivable	289,899,000		293,667,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	1,095,000		1,026,000
Unpaid Principal Balance	1,467,000		1,347,000
Specific Allowance	20,000		25,000
Loans without a specific allowance [Abstract]
Recorded Balance	2,743,000		2,239,000
Unpaid Principal Balance	2,800,000		2,664,000
Total Loans [Abstract]
Recorded Balance	3,838,000		3,265,000
Unpaid Principal Balance	4,267,000		4,011,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	3,891,000	2,100,000
Interest income recognized	8,000	11,000
Loans Receivable, Modifications, Still Accruing Interest	730,000	$ 597,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	3,108,000		2,687,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	943,000		874,000
Financing Receivable Modifications Performing Recorded Investment	$ 730,000		578,000
Recorded Balance of Troubled Debt Restructurings [Abstract]
Financing Receivables, Modifications during Period, Number	1	0
Financing Receivables, Modifications during Period, Balance	$ 161,000	$ 0
1-4 Family residential properties | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	277,265,000		282,441,000
1-4 Family residential properties | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	4,081,000		2,654,000
1-4 Family residential properties | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	8,553,000		8,572,000
1-4 Family residential properties | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Multifamily residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Current	60,881,000		61,798,000
Total Loans Receivable	60,881,000		61,798,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	307,000		313,000
Unpaid Principal Balance	307,000		313,000
Specific Allowance	0		0
Loans without a specific allowance [Abstract]
Recorded Balance	0		55,000
Unpaid Principal Balance	0		55,000
Total Loans [Abstract]
Recorded Balance	307,000		368,000
Unpaid Principal Balance	307,000		368,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	308,000	4,155,000
Interest income recognized	0	43,000
Loans Receivable, Modifications, Still Accruing Interest		3,407,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	307,000		368,000
Multifamily residential properties | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	59,875,000		60,954,000
Multifamily residential properties | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	470,000		476,000
Multifamily residential properties | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	536,000		368,000
Multifamily residential properties | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Commercial real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	3,412,000		3,694,000
Current	695,730,000		678,063,000
Total Loans Receivable	699,142,000		681,757,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	5,732,000		5,544,000
Unpaid Principal Balance	5,732,000		5,565,000
Specific Allowance	470,000		531,000
Loans without a specific allowance [Abstract]
Recorded Balance	163,000		303,000
Unpaid Principal Balance	248,000		368,000
Total Loans [Abstract]
Recorded Balance	5,895,000		5,847,000
Unpaid Principal Balance	5,980,000		5,933,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	5,993,000	13,434,000
Interest income recognized	3,000	0
Loans Receivable, Modifications, Still Accruing Interest	249,000	2,121,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	5,646,000		5,596,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	1,362,000		1,376,000
Financing Receivable Modifications Performing Recorded Investment	249,000		251,000
Commercial real estate | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	665,007,000		647,208,000
Commercial real estate | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	17,007,000		16,941,000
Commercial real estate | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	17,128,000		17,608,000
Commercial real estate | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Agricultural loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	537,000		190,000
Current	73,805,000		86,441,000
Total Loans Receivable	74,342,000		86,631,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	201,000		212,000
Unpaid Principal Balance	986,000		1,009,000
Specific Allowance	5,000		2,000
Loans without a specific allowance [Abstract]
Recorded Balance	736,000		545,000
Unpaid Principal Balance	203,000		0
Total Loans [Abstract]
Recorded Balance	937,000		757,000
Unpaid Principal Balance	1,189,000		1,009,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	885,000	1,398,000
Interest income recognized	0	0
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	937,000		757,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	734,000		757,000
Agricultural loans | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	72,019,000		83,469,000
Agricultural loans | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	1,238,000		2,304,000
Agricultural loans | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	1,085,000		858,000
Agricultural loans | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Commercial and industrial loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	4,974,000		965,000
Current	453,717,000		443,298,000
Total Loans Receivable	458,691,000		444,263,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	5,827,000		5,774,000
Unpaid Principal Balance	6,315,000		6,037,000
Specific Allowance	21,000		64,000
Loans without a specific allowance [Abstract]
Recorded Balance	737,000		909,000
Unpaid Principal Balance	768,000		1,249,000
Total Loans [Abstract]
Recorded Balance	6,564,000		6,683,000
Unpaid Principal Balance	7,083,000		7,286,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	7,056,000	4,096,000
Interest income recognized	2,000	2,000
Loans Receivable, Modifications, Still Accruing Interest	20,000	37,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	6,544,000		6,658,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	5,029,000		5,690,000
Financing Receivable Modifications Performing Recorded Investment	20,000		25,000
Commercial and industrial loans | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	448,416,000		425,846,000
Commercial and industrial loans | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	3,290,000		11,492,000
Commercial and industrial loans | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	6,985,000		6,925,000
Commercial and industrial loans | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Consumer loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	201,000		272,000
Current	28,583,000		29,477,000
Total Loans Receivable	28,784,000		29,749,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	170,000		200,000
Unpaid Principal Balance	170,000		200,000
Specific Allowance	0		1,000
Loans without a specific allowance [Abstract]
Recorded Balance	113,000		102,000
Unpaid Principal Balance	144,000		119,000
Total Loans [Abstract]
Recorded Balance	283,000		302,000
Unpaid Principal Balance	314,000		319,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	294,000	406,000
Interest income recognized	0	0
Loans Receivable, Modifications, Still Accruing Interest		3,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	283,000		302,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	170,000		201,000
Consumer loans | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	28,358,000		29,375,000
Consumer loans | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	3,000		5,000
Consumer loans | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	423,000		369,000
Consumer loans | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
All other loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Current	134,318,000		106,859,000
Total Loans Receivable	134,318,000		106,859,000
Total Loans 90 Days & Accruing	0		0
All other loans | Pass
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	131,290,000		103,339,000
All other loans | Special Mention
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	3,028,000		3,520,000
All other loans | Substandard
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
All other loans | Doubtful
Loans Receivable Aging Analysis [Abstract]
Total Loans Receivable	0		0
Loans secured by real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	9,032,000		9,492,000
Current	1,272,530,000		1,262,507,000
Total Loans Receivable	1,281,562,000		1,271,999,000
Total Loans 90 Days & Accruing	0		0
Loans with a specific allowance [Abstract]
Recorded Balance	7,134,000		7,159,000
Unpaid Principal Balance	7,506,000		7,501,000
Specific Allowance	490,000		556,000
Loans without a specific allowance [Abstract]
Recorded Balance	2,951,000		2,612,000
Unpaid Principal Balance	3,093,000		3,102,000
Total Loans [Abstract]
Recorded Balance	10,085,000		9,771,000
Unpaid Principal Balance	10,599,000		10,603,000
Average recorded investment and interest income recognized [Abstract]
Average investment in impaired loans	10,238,000	19,850,000
Interest income recognized	11,000	$ 54,000
Balances of Nonaccrual Loans [Abstract]
Recorded balance of nonaccrual loans	9,106,000		8,942,000
Troubled Debt Restructuring [Abstract]
Troubled Debt Restructurings Balance	2,305,000		2,250,000
Financing Receivable Modifications Performing Recorded Investment	979,000		829,000
Financing Receivables, 30 to 59 Days Past Due [Member]
Loans Receivable Aging Analysis [Abstract]
Total Past Due	8,199,000		3,509,000
Financing Receivables, 30 to 59 Days Past Due [Member] | Construction and land development
Loans Receivable Aging Analysis [Abstract]
Total Past Due	205,000		26,000
Financing Receivables, 30 to 59 Days Past Due [Member] | Agricultural real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,103,000		0
Financing Receivables, 30 to 59 Days Past Due [Member] | 1-4 Family residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	2,289,000		3,023,000
Financing Receivables, 30 to 59 Days Past Due [Member] | Multifamily residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,036,000		90,000
Financing Receivables, 30 to 59 Days Past Due [Member] | Agricultural loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	369,000		0
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial and industrial loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	3,101,000		192,000
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	96,000		178,000
Financing Receivables, 30 to 59 Days Past Due [Member] | All other loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, 30 to 59 Days Past Due [Member] | Loans secured by real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	4,633,000		3,139,000
Financing Receivables, 60 to 89 Days Past Due [Member]
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,999,000		726,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Construction and land development
Loans Receivable Aging Analysis [Abstract]
Total Past Due	253,000		48,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Agricultural real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, 60 to 89 Days Past Due [Member] | 1-4 Family residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	244,000		538,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Multifamily residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	260,000		38,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Agricultural loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	10,000		32,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial and industrial loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,139,000		3,000
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	93,000		67,000
Financing Receivables, 60 to 89 Days Past Due [Member] | All other loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, 60 to 89 Days Past Due [Member] | Loans secured by real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	757,000		624,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans Receivable Aging Analysis [Abstract]
Total Past Due	4,546,000		6,684,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Construction and land development
Loans Receivable Aging Analysis [Abstract]
Total Past Due	45,000		0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Agricultural real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		396,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | 1-4 Family residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	1,481,000		1,767,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Multifamily residential properties
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	2,116,000		3,566,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Agricultural loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	158,000		158,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and industrial loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	734,000		770,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	12,000		27,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | All other loans
Loans Receivable Aging Analysis [Abstract]
Total Past Due	0		0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans secured by real estate
Loans Receivable Aging Analysis [Abstract]
Total Past Due	$ 3,642,000		$ 5,729,000