Loans - Changes in the Allowance for Losses (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance			$ 10,454,000
(Release of)/provision for losses	$ 653,000	$ 760,000	4,542,000	$ 1,074,000
Ending balance	15,821,000		$ 15,821,000		$ 10,454,000
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Farm & Ranch
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	6,039,000	7,264,000	$ 10,454,000	7,017,000	$ 7,017,000
(Release of)/provision for losses	(300,000)	760,000	(412,000)	1,074,000
Charge-offs	0	0	(394,000)	(67,000)
Ending balance	5,739,000	8,024,000	$ 5,739,000	8,024,000	10,454,000
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Farm & Ranch | Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Release of)/provision for losses			$ 1,800,000
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	0	(3,909,000)	0	0
Ending balance					(3,909,000)
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	6,039,000	7,264,000	6,545,000	7,017,000	7,017,000
Ending balance					6,545,000
Rural  Utilities
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	8,900,000	0	0	0	0
(Release of)/provision for losses	1,182,000	0	4,704,000	0
Charge-offs	0	0	0	0
Ending balance	10,082,000	0	$ 10,082,000	0	0
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Rural  Utilities | Collateral Pledged
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Release of)/provision for losses			$ 0
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	0	5,378,000	0	0
Ending balance					5,378,000
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 8,900,000	$ 0	$ 5,378,000	$ 0	0
Ending balance					$ 5,378,000