Accounting Policies - Cash and Cash Equivalents and Statements of Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Other Significant Noncash Transactions [Line Items]
SEC Schedule III, Real Estate, Acquisitions Through Foreclosures	$ 0	$ 5,261
Proceeds from sale of Farmer Mac Guaranteed Securities	196,290	247,975
Consolidation of guaranteed securities from off-balance sheet to Loans Held for Investment and Debt Securities of consolidated trusts held by third parties	116,983	161,880
Purchases of Investment Securities Traded, Not Yet Settled	$ 48,600	$ 50,000