Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net interest income	$ 6,742	$ 5,347
Fee and other income	87	120
Net revenues	6,829	5,467
Investment gains, net	45	(158)
Fair value gains (losses), net	784	(276)
Administrative expenses	(748)	(749)
Benefit (provision) for credit losses	765	(2,583)
Foreclosed property income (expense)	5	(80)
Credit-related income (expense)	770	(2,663)
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	(731)	(637)
Credit enhancement expense	(284)	(376)
Change in expected credit enhancement recoveries	(31)	188
Other expenses, net	(319)	(218)
Income before federal income taxes	6,315	578
Provision for federal income taxes	(1,322)	(117)
Net income	4,993	461
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	(731)	(637)
Business Segments [Member] | FNM_SingleFamily
Segment Reporting Information [Line Items]
Net interest income	5,894	4,541
Fee and other income	62	94
Net revenues	5,956	4,635
Investment gains, net	64	(152)
Fair value gains (losses), net	740	(460)
Administrative expenses	(623)	(629)
Benefit (provision) for credit losses	662	(2,172)
Foreclosed property income (expense)	17	(78)
Credit-related income (expense)	679	(2,250)
Credit enhancement expense	(226)	(316)
Change in expected credit enhancement recoveries	(16)	58
Other expenses, net	(287)	(163)
Income before federal income taxes	5,556	86
Provision for federal income taxes	(1,162)	(18)
Net income	4,394	68
Business Segments [Member] | FNM_SingleFamily | US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	(731)	(637)
Business Segments [Member] | fnm_MultifamilyMember
Segment Reporting Information [Line Items]
Net interest income	848	806
Fee and other income	25	26
Net revenues	873	832
Investment gains, net	(19)	(6)
Fair value gains (losses), net	44	184
Administrative expenses	(125)	(120)
Benefit (provision) for credit losses	103	(411)
Foreclosed property income (expense)	(12)	(2)
Credit-related income (expense)	91	(413)
Credit enhancement expense	(58)	(60)
Change in expected credit enhancement recoveries	(15)	130
Other expenses, net	(32)	(55)
Income before federal income taxes	759	492
Provision for federal income taxes	(160)	(99)
Net income	599	393
Business Segments [Member] | fnm_MultifamilyMember | US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	$ 0	$ 0