Guarantees - Changes in Reserve for Losses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance		$ 2,164
(Release of)/provision for losses	$ (3)	250	$ (3)	$ 97
Ending balance	2,164	3,277	2,164
Farm & Ranch
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance		2,164	2,167	2,070
(Release of)/provision for losses		81	(3)	97
Ending balance	2,164	2,097	2,164	2,167
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance		(148)
Ending balance	(148)		(148)
Rural Utilities:
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance		0	0	0
(Release of)/provision for losses		169	0	0
Ending balance	0	1,180	0	$ 0
Rural Utilities: | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance		$ 1,011
Ending balance	$ 1,011		$ 1,011