Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Available-for-sale Securities (Details) - Farmer Mac Guaranteed Securities and USDA Securities [Member] $ in Thousands	Sep. 30, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,019,282
Available-for-sale Securities, Due after one year through five years, Amortized Cost	2,664,743
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	777,186
Available-for-sale Securities, Due after ten years, Amortized Cost	1,552,846
Available-for-sale Securities, Amortized Cost Basis	6,014,057
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,018,343
Available-for-sale Securities, Due after one year through five years, Fair Value	2,652,381
Available-for-sale Securities, Due after five years through ten years, Fair Value	751,727
Available-for-sale Securities, Due after ten years, Fair Value	1,534,853
Available-for-sale, at fair value	$ 5,957,304
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	2.58%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	2.95%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	3.14%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.64%
Available-for-sale Securities, Total Weighted-Average Yield	2.84%

Available-for-sale Securities, Debt Maturities, after Ten Years, Weighted-Average Yield

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Available-for-sale Securities, Debt Maturities, Next Twelve Months, Weighted-Average Yield

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Available-for-sale Securities, Debt Maturities, Weighted-Average Yield

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Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Weighted-Average Yield

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Available-for-sale Securities, Debt Maturities, Year Two Through Five, Weighted-Average Yield

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Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB
-Name Accounting Standards Codification
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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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