Related Party Transactions Zions (Details) (USD $)	3 Months Ended							12 Months Ended			15 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012
Related Party Transaction [Line Items]
Guarantee fees	$ 6,568,000	$ 6,401,000	$ 6,064,000	$ 5,930,000	$ 5,966,000	$ 6,320,000	$ 6,387,000	$ 24,963,000	$ 24,821,000	$ 24,091,000	$ 6,148,000
Derivative, Notional Amount	6,815,268,000				6,586,530,000			6,815,268,000	6,586,530,000
Accrued interest payable	51,779,000				60,854,000			51,779,000	60,854,000
Zions First National Bank [Member]
Related Party Transaction [Line Items]
Guarantee fees								633,000	940,000	1,210,000
Expense Related to Distribution or Servicing and Underwriting Fees								1,980,000	1,870,000	1,678,000
Related Party's percent of Farmer Mac Total Outstanding Book of Business	4.70%				4.70%			4.70%	4.70%
Accrued interest payable	500,000				1,000,000			500,000	1,000,000
Notes issued through related party dealer								0	37,000,000	220,900,000
Mortgage Loans on Real Estate [Member] | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party								168,826,000	125,088,000	176,288,000
USDA Guarantee | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party								14,415,000	5,218,000	5,775,000
Sale of Farmer Mac Guaranteed Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction								5,327,000	0	5,695,000
Farmer Mac I [Member] | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Farmer Mac I Purchases from Related Party, percent of total purchases with the related party								29.60%	25.20%	46.10%
Related Party Transaction, Related Party's percent of total Program								8.80%	4.50%	11.40%
USDA Guarantee | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's percent of total Program								3.00%	1.30%
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Related Party Transaction [Line Items]
Derivative, Notional Amount	805,622,000				1,906,123,000			805,622,000	1,906,123,000
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member] | Zions First National Bank [Member]
Related Party Transaction [Line Items]
Derivative, Notional Amount	$ 49,300,000				$ 72,700,000			$ 49,300,000	$ 72,700,000