Guarantees - Changes in Reserve for Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 2,164
Provision for/(release of) losses	393	$ (129)
Ending balance	3,420
Farm & Ranch
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	2,164	2,167
Provision for/(release of) losses	4	(129)
Charge-offs	0	0
Ending balance	2,020	2,038
Rural  Utilities
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	0
Provision for/(release of) losses	389	0
Charge-offs	0	0
Ending balance	$ 1,400	$ 0