Related Party Transactions - Zions (Details) - Zions First National Bank [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Related Party's percent of Farmer Mac Total Outstanding Book of Business	5.00%	5.30%
Related Party Transaction, Expenses from Transactions with Related Party	$ 11,500	$ 9,900	$ 9,300
Sale of Farmer Mac Guaranteed Securities [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	$ 128,924	$ 273,586	$ 255,338
Common Class A, Voting [Member]
Related Party Transaction [Line Items]
Common Stock Ownership Percentage by a Related Party	31.20%
Farm & Ranch [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's Percent of Total Program Loans	11.20%	15.90%	23.90%
Related Party Transaction, Related Party's percent of total Program	7.50%	11.20%	15.20%
USDA Guarantee [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's percent of total Program	3.80%	3.40%	3.60%
Mortgages [Member] | Farm & Ranch [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 126,449	$ 153,140	$ 178,890
USDA Securities [Member] | USDA Guarantee [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 20,368	$ 16,600	$ 13,718