Accounting Policies - Cash and Cash Equivalents and Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 01, 2016
Other Significant Noncash Transactions [Line Items]
Interest Paid	$ 161,060	$ 110,609	$ 108,254
Income Taxes Paid	39,500	29,500	31,000
SEC Schedule III, Real Estate, Acquisitions Through Foreclosures	5,400	0	0
Proceeds from sale of Farmer Mac Guaranteed Securities	519,219	609,347	336,913
Consolidation of guaranteed securities from off-balance sheet to Loans Held for Investment and Debt Securities of consolidated trusts held by third parties	363,475	511,393	336,913
Purchases of Investment Securities Traded, Not Yet Settled	1,400	0	20,000
Preferred Stock Redemption Premium	0	0	8,147
Unsettled Class C Common Stock Repurchases	0	0	197
Transfer of USDA Guaranteed Securities- Available-for-sale to Held-to-maturity; Unrealized holding gains				$ 73,100
Transfer of Farmer Mac Guaranteed Securities - Available-for-sale to Held-to-maturity; Unrealized holding gains				700
Transfer of Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Available-for-sale to Held-to-maturity; Unrealized holding gains				$ 73,800
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Other Significant Noncash Transactions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		2,013,151
Fair Value, Inputs, Level 3 [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Other Significant Noncash Transactions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		1,980,327
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Other Significant Noncash Transactions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	32,824	0
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Other Significant Noncash Transactions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0	1,980,327	$ 0
USDA Guarantee [Member] | Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Other Significant Noncash Transactions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		$ 2,000,000