Accounting Policies - Cash and Cash Equivalents and Statements of Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Other Significant Noncash Transactions [Line Items]
SEC Schedule III, Real Estate, Acquisitions Through Foreclosures	$ 128	$ 5,261
Proceeds from sale of Farmer Mac Guaranteed Securities	305,391	404,246
Consolidation of guaranteed securities from off-balance sheet to Loans Held for Investment and Debt Securities of consolidated trusts held by third parties	199,764	277,307
Purchases of Investment Securities Traded, Not Yet Settled	$ 248,600	$ 9,987