Condensed Consolidated Balance Sheets (Parenthetical) (Unaudited) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Restricted cash (includes $36,921 and $27,515, respectively, related to consolidated trusts)	$ 41,439	$ 32,542
Accrued interest receivable, net (includes $7,259 and $7,169, respectively, related to consolidated trusts)	7,990us-gaap_InterestReceivable	8,193us-gaap_InterestReceivable
Investments in securities:
Available-for-sale, at fair value (includes $468 and $596, respectively, related to consolidated trusts)	27,732	30,654
Liabilities:
Accrued interest payable (includes $8,223 and $8,282, respectively, related to consolidated trusts)	10,324	10,232
Other liabilities (includes $437 and $503, respectively, related to consolidated trusts)	11,475us-gaap_OtherLiabilities	12,019us-gaap_OtherLiabilities
Fannie Mae stockholders’ equity:
Senior preferred stock, 1,000,000 shares issued	1,000,000fnm_SeniorPreferredStockIssuedShares	1,000,000fnm_SeniorPreferredStockIssuedShares
Senior preferred stock, 1,000,000 shares outstanding	1,000,000	1,000,000
Preferred stock, 700,000,000 shares are authorized	700,000,000us-gaap_PreferredStockSharesAuthorized	700,000,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, 555,374,922 shares issued	555,374,922us-gaap_PreferredStockSharesIssued	555,374,922us-gaap_PreferredStockSharesIssued
Preferred stock, 555,374,922 shares outstanding	555,374,922us-gaap_PreferredStockSharesOutstanding	555,374,922us-gaap_PreferredStockSharesOutstanding
Common stock, 1,308,762,703 shares issued	1,308,762,703us-gaap_CommonStockSharesIssued	1,308,762,703us-gaap_CommonStockSharesIssued
Common stock, 1,158,082,750 shares outstanding	1,158,082,750us-gaap_CommonStockSharesOutstanding	1,158,082,750us-gaap_CommonStockSharesOutstanding
Treasury stock, at cost, 150,679,953 shares	150,679,953us-gaap_TreasuryStockShares	150,679,953us-gaap_TreasuryStockShares
Fannie Mae [Member]
Liabilities:
Debt at fair value	7,920	6,403
Consolidated Trusts [Member]
Assets:
Restricted cash (includes $36,921 and $27,515, respectively, related to consolidated trusts)	36,921	27,515
Accrued interest receivable, net (includes $7,259 and $7,169, respectively, related to consolidated trusts)	7,259	7,169
Investments in securities:
Available-for-sale, at fair value (includes $468 and $596, respectively, related to consolidated trusts)	468	596
Mortgage loans:
Mortgage loans held for investment, at fair value	15,570	15,629
Liabilities:
Accrued interest payable (includes $8,223 and $8,282, respectively, related to consolidated trusts)	8,223	8,282
Debt at fair value	21,373	19,483
Other liabilities (includes $437 and $503, respectively, related to consolidated trusts)	$ 437	$ 503

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