Loans and Allowance for Loan Losses Loans and Allowance for Loan Losses - Allowance (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) alternative	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 08, 2016 USD ($)
Financing Receivable, Impaired [Line Items]
Minimum balance of adversely classified loan in which a detailed analysis is performed	$ 250,000
Number of alternatives for measuring impaired loans receivable | alternative	3
Balance, beginning of year	$ 16,753,000	$ 14,576,000	$ 13,682,000
Provision for loan losses	7,462,000	2,826,000	1,318,000
Losses charged off	(5,195,000)	(1,675,000)	(906,000)
Recoveries	957,000	1,026,000	482,000
Balance, end of period	19,977,000	16,753,000	14,576,000
Individually evaluated for impairment	614,000	858,000	134,000
Collectively evaluated for impairment	19,354,000	15,881,000	14,442,000
Ending balance	1,939,501,000	1,825,992,000	1,281,889,000
Individually evaluated for impairment	13,086,000	5,266,000	1,174,000
Collectively evaluated for impairment	$ 1,926,156,000	1,812,640,000	1,280,715,000
Financing Receivable, Net				$ 8,688,000
Unsecured Open-end Loans [Member]
Financing Receivable, Impaired [Line Items]
Period When Loans Are Charged Down	180 days
Other Secured Loans [Member]
Financing Receivable, Impaired [Line Items]
Period When Loans Are Charged Down	120 days
Commercial/Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Balance, beginning of year	$ 12,901,000	11,379,000	10,914,000
Provision for loan losses	6,884,000	1,467,000	451,000
Losses charged off	(3,795,000)	(747,000)	(289,000)
Recoveries	556,000	802,000	303,000
Balance, end of period	16,546,000	12,901,000	11,379,000
Individually evaluated for impairment	586,000	192,000	134,000
Collectively evaluated for impairment	15,951,000	12,695,000	11,245,000
Ending balance	1,371,787,000	1,204,799,000	807,736,000
Individually evaluated for impairment	11,372,000	1,956,000	744,000
Collectively evaluated for impairment	1,360,156,000	1,199,003,000	806,992,000
Agriculture/Agricultural Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Balance, beginning of year	2,249,000	1,337,000	1,360,000
Provision for loan losses	153,000	933,000	(25,000)
Losses charged off	(662,000)	(30,000)	0
Recoveries	2,000	9,000	2,000
Balance, end of period	1,742,000	2,249,000	1,337,000
Individually evaluated for impairment	2,000	660,000	0
Collectively evaluated for impairment	1,740,000	1,589,000	1,337,000
Ending balance	213,521,000	212,513,000	198,066,000
Individually evaluated for impairment	488,000	1,345,000	430,000
Collectively evaluated for impairment	213,033,000	211,168,000	197,636,000
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Balance, beginning of year	874,000	994,000	790,000
Provision for loan losses	100,000	113,000	267,000
Losses charged off	(217,000)	(234,000)	(64,000)
Recoveries	129,000	1,000	1,000
Balance, end of period	886,000	874,000	994,000
Individually evaluated for impairment	25,000	6,000	0
Collectively evaluated for impairment	861,000	868,000	994,000
Ending balance	315,123,000	366,823,000	232,348,000
Individually evaluated for impairment	1,026,000	1,752,000	0
Collectively evaluated for impairment	$ 314,097,000	360,825,000	232,348,000
1-4 Family residential properties
Financing Receivable, Impaired [Line Items]
Period When Loans Are Charged Down	180 days
Consumer loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of year	$ 693,000	642,000	386,000
Provision for loan losses	361,000	501,000	633,000
Losses charged off	(521,000)	(664,000)	(553,000)
Recoveries	270,000	214,000	176,000
Balance, end of period	803,000	693,000	642,000
Individually evaluated for impairment	1,000	0	0
Collectively evaluated for impairment	802,000	693,000	642,000
Ending balance	39,070,000	41,857,000	43,739,000
Individually evaluated for impairment	200,000	213,000	0
Collectively evaluated for impairment	38,870,000	41,644,000	43,739,000
Unallocated [Member]
Financing Receivable, Impaired [Line Items]
Balance, beginning of year	36,000	224,000	232,000
Provision for loan losses	(36,000)	(188,000)	(8,000)
Losses charged off	0	0	0
Recoveries	0	0	0
Balance, end of period	0	36,000	224,000
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	0	36,000	224,000
Ending balance	0	0	0
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	0	0	$ 0
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	9,000	14,000
Financing Receivable, Net	259,000	8,086,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial/Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	9,000
Financing Receivable, Net	259,000	3,840,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Agriculture/Agricultural Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	0
Financing Receivable, Net	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	0
Financing Receivable, Net	0	4,246,000
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer loans
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	0
Financing Receivable, Net	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Unallocated [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Allowance for Credit Losses	0
Financing Receivable, Net	$ 0	$ 0