Loans - Changes in the Allowance for Losses (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 13,832,000
(Release of)/provision for losses	913,000	$ 3,438,000
Ending balance	14,807,000		$ 13,832,000
Farm & Ranch
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	3,745,000	10,454,000	10,454,000
(Release of)/provision for losses	(27,000)	808,000
Charge-offs	0	0	0
Ending balance	3,718,000	7,353,000	3,745,000
Farm & Ranch | Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Release of)/provision for losses	0	2,200,000
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	(3,909,000)	(3,909,000)
Ending balance			0
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	3,745,000	6,545,000	6,545,000
Ending balance			3,745,000
Rural Utilities
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	10,087,000	0	0
(Release of)/provision for losses	1,002,000	2,125,000
Charge-offs	0	0	0
Ending balance	11,089,000	7,503,000	10,087,000
Rural Utilities | Collateral Pledged
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Release of)/provision for losses	0	0
Rural Utilities | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	5,378,000	5,378,000
Ending balance			0
Rural Utilities | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 10,087,000	$ 5,378,000	5,378,000
Ending balance			$ 10,087,000