Loans Receivable and the Allowance for Credit Losses - Activity in Allowance for Credit Losses (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 51,187,000	$ 29,652,000	$ 29,079,000	$ 29,307,000	$ 29,307,000
Charge-offs	(2,103,000)	(2,187,000)	(3,600,000)	(3,542,000)
Recoveries	236,000	530,000	535,000	636,000
Credit loss expense	6,324,000	696,000	25,646,000	2,290,000
Ending balance	55,644,000	28,691,000	$ 55,644,000	28,691,000	$ 29,079,000
Accounting standards update			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Credit loss expense, off balance sheet credit exposure	(1,600,000)		$ 772,000	0
Loans receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Accrued interest receivable	15,300,000		15,300,000
Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance			3,984,000
Ending balance					$ 3,984,000
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1,146,000	3,691,000	3,748,000	3,637,000	3,637,000
Charge-offs	(109,000)	(16,000)	(193,000)	(150,000)
Recoveries	1,000	2,000	26,000	9,000
Credit loss expense	370,000	43,000	384,000	224,000
Ending balance	1,408,000	3,720,000	1,408,000	3,720,000	3,748,000
Agricultural | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(2,557,000)		(2,557,000)
Ending balance					(2,557,000)
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19,309,000	7,594,000	8,394,000	7,478,000	7,478,000
Charge-offs	(902,000)	(1,759,000)	(1,373,000)	(2,113,000)
Recoveries	166,000	100,000	379,000	148,000
Credit loss expense	136,000	1,698,000	8,581,000	2,120,000
Ending balance	18,709,000	7,633,000	18,709,000	7,633,000	8,394,000
Commercial and industrial | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2,728,000		2,728,000
Ending balance					2,728,000
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23,138,000	14,798,000	13,804,000	15,635,000	15,635,000
Charge-offs	(792,000)	(310,000)	(1,512,000)	(960,000)
Recoveries	11,000	257,000	19,000	265,000
Credit loss expense	5,864,000	(1,090,000)	14,610,000	(1,285,000)
Ending balance	28,221,000	13,655,000	28,221,000	13,655,000	13,804,000
Commercial real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1,300,000		1,300,000
Ending balance					1,300,000
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6,425,000	3,272,000	2,685,000	2,349,000	2,349,000
Charge-offs	(103,000)	(1,000)	(103,000)	(50,000)
Recoveries	8,000	8,000	15,000	17,000
Credit loss expense	(256,000)	98,000	1,427,000	1,061,000
Ending balance	6,074,000	3,377,000	6,074,000	3,377,000	2,685,000
Residential real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2,050,000		2,050,000
Ending balance					2,050,000
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1,169,000	297,000	448,000	208,000	208,000
Charge-offs	(197,000)	(101,000)	(419,000)	(269,000)
Recoveries	50,000	163,000	96,000	197,000
Credit loss expense	210,000	(53,000)	644,000	170,000
Ending balance	1,232,000	$ 306,000	1,232,000	$ 306,000	448,000
Consumer | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 463,000		$ 463,000
Ending balance					$ 463,000