Guarantees - Reserve For Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Jan. 01, 2020	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Reserve for losses	$ 3,568		$ 3,568		$ 2,164
Provision for/(release of) losses	547	$ (137)	$ 540	$ (424)
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Farm & Ranch
Guarantor Obligations [Line Items]
Reserve for losses	2,278	1,743	$ 2,278	1,743	$ 2,164	$ 1,650	$ 2,016	$ 1,880	$ 2,167	$ 2,167
Provision for/(release of) losses	628	(137)	$ 262	(424)
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Rural  Utilities
Guarantor Obligations [Line Items]
Reserve for losses	1,290	0	$ 1,290	0	0	$ 1,370	$ 1,011	$ 0	$ 0	$ 0
Provision for/(release of) losses	(80)	$ 0	$ 279	$ 0
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
LTSPCs and Farmer Mac Guaranteed Securities | Farm & Ranch
Guarantor Obligations [Line Items]
Reserve for losses	2,278		$ 2,278		2,164
LTSPCs | Rural  Utilities
Guarantor Obligations [Line Items]
Reserve for losses	$ 1,290		$ 1,290		$ 0