Related Party Transactions - National Rural Utilities Cooperative Financial Corporation (Details) - CFC [Member] - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's percent of total Program	10.30%	16.70%	40.60%
Related Party Transaction, Purchases from Related Party	$ 487,341,000	$ 741,895,000	$ 1,310,599,000
Related Party's percent of Farmer Mac Total Outstanding Book of Business	24.60%	25.70%
Related Party Transaction, Other Revenues from Transactions with Related Party	$ 65,000	$ 74,000	82,000
Due from Related Parties	5,200,000	3,200,000
Interest Income, Related Party	43,900,000	27,600,000	15,900,000
Accounts Receivable, Related Parties, Current	178,000	190,871
Revenue from Related Parties	2,200,000	2,000,000	500,000
Related Party Transaction, Expenses from Transactions with Related Party	$ 3,500,000	$ 3,300,000	$ 3,300,000
Rural Utilities [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's percent of total Program	100.00%
Institutional Credit [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Related Party's percent of total Program	14.70%	11.90%	65.20%
Real Estate Loan [Member] \| Rural Utilities [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 137,341,000	$ 50,491,000	$ 108,337,000
Guarantee Obligations [Member] \| Rural Utilities [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	350,000,000	250,000,000	380,000,000
Off-balance sheet [Member] \| Farmer Mac Guaranteed Securities [Member] \| Institutional Credit [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	0	0	300,000,000
Off-balance sheet [Member] \| Revolving Credit Facility [Member] \| Institutional Credit [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 0	$ 441,404,000	$ 522,262,000
Common Class A, Voting [Member]
Related Party Transaction [Line Items]
Common Stock Ownership Percentage by a Related Party	7.90%

Common Stock Ownership Percentage by a Related Party

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Related Party Transaction, Related Party's percent of total Program

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Related Party's percent of Farmer Mac Total Outstanding Book of Business

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Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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Amount of interest income earned from interest-bearing assets owed to the entity by related party.

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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

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Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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