Notes Payable - Earliest Interest Rate Reset Date of Borrowings Outstanding (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 7,353,953,000us-gaap_ShortTermBorrowings	$ 7,338,781,000us-gaap_ShortTermBorrowings
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,569,961,000	1,411,140,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,219,192,000	1,290,629,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	682,224,000	528,322,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	872,576,000	675,968,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,127,233,000	1,095,110,000
Debt and Capital Lease Obligations	12,825,139,000us-gaap_DebtAndCapitalLeaseObligations	12,339,950,000us-gaap_DebtAndCapitalLeaseObligations
Short-term Debt, Weighted Average Interest Rate	0.32%	0.64%
Long-term Debt, Weighted Average Interest Rate	1.56%	1.60%
Debt, Weighted Average Interest Rate	0.85%us-gaap_DebtWeightedAverageInterestRate	1.03%us-gaap_DebtWeightedAverageInterestRate
Meduim Term Notes Called During the Period	500,000,000agm_MeduimTermNotesCalledDuringPeriod	400,000,000agm_MeduimTermNotesCalledDuringPeriod
Due within one year [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	8,887,288,000
Short-term Debt, Weighted Average Interest Rate	0.44%
Due year two [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,391,014,000
Long-term Debt, Weighted Average Interest Rate	1.26%
Due year three [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Three	711,299,000
Long-term Debt, Weighted Average Interest Rate	1.19%
Due year four [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Four	532,279,000
Long-term Debt, Weighted Average Interest Rate	1.38%
Due year five [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Five	470,183,000
Long-term Debt, Weighted Average Interest Rate	1.76%
Due after five years [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 833,076,000
Long-term Debt, Weighted Average Interest Rate	3.29%

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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after 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 of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third 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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Weighted average interest rate of long-term debt outstanding.

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