Fair Value - Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 141	$ 136	$ 95	$ 46
Total gains (losses) (realized/unrealized) Included in Net Income	(13)	(4)	(18)	(6)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	15	0	18	0
Sales	0	(95)	0	(95)
Issues	0	0	0	0
Settlements	0	0	0	(3)
Transfers out of Level 3	(51)	(3)	(91)	(49)
Transfers into Level 3	37	27	125	168
Ending Balance	129	61	129	61
Net unrealized gain (losses) related to assets	0	0	1	(8)
Total trading securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	141	38	94	45
Total gains (losses) (realized/unrealized) Included in Net Income	(13)	0	(18)	(9)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	15	0	18	0
Sales	0	(1)	0	(1)
Issues	0	0	0	0
Settlements	0	0	0	0
Transfers out of Level 3	(51)	(3)	(90)	(48)
Transfers into Level 3	37	26	125	73
Ending Balance	129	60	129	60
Net unrealized gain (losses) related to assets	0	0	1	(8)
Total trading securities | Other agency
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	1	1
Total gains (losses) (realized/unrealized) Included in Net Income		0	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)		0	0	0
Purchases		0	0	0
Sales		0	0	0
Issues		0	0	0
Settlements		0	0	0
Transfers out of Level 3		0	(1)	(1)
Transfers into Level 3		1	0	1
Ending Balance	0	1	0	1
Net unrealized gain (losses) related to assets		0	0	0
Total trading securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		98		0
Total gains (losses) (realized/unrealized) Included in Net Income		(4)		3
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)		0		0
Purchases		0		0
Sales		(94)		(94)
Issues		0		0
Settlements		0		(3)
Transfers out of Level 3		0		0
Transfers into Level 3		0		94
Ending Balance		0		0
Net unrealized gain (losses) related to assets		0		0
Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	452	980	648	792
Total gains (losses) (realized/unrealized) Included in Net Income	3	6	11	(11)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(6)	7	(4)	9
Purchases	0	0	0	342
Sales	0	0	0	(342)
Issues	0	0	0	0
Settlements	(29)	(48)	(128)	(125)
Transfers out of Level 3	0	(218)	(107)	(218)
Transfers into Level 3	376	0	376	280
Ending Balance	796	727	796	727
Net unrealized gain (losses) related to assets	0	0	0	0
Total available-for-sale securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	84	446	195	171
Total gains (losses) (realized/unrealized) Included in Net Income	0	1	1	1
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	2	2	4	2
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(12)	(9)	(19)	(12)
Transfers out of Level 3	0	(218)	(107)	(218)
Transfers into Level 3	376	0	376	278
Ending Balance	450	222	450	222
Net unrealized gain (losses) related to assets	0	0	0	0
Total available-for-sale securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	2	2	2	0
Total gains (losses) (realized/unrealized) Included in Net Income	0	0	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(2)	0	(2)	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	0	0	0	0
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	2
Ending Balance	0	2	0	2
Net unrealized gain (losses) related to assets	0	0	0	0
Total available-for-sale securities | Mortgage revenue bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	172	278	216	315
Total gains (losses) (realized/unrealized) Included in Net Income	2	0	2	(3)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(2)	0	(4)	3
Purchases	0	0	0	74
Sales	0	0	0	(74)
Issues	0	0	0	0
Settlements	(8)	(23)	(50)	(60)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	164	255	164	255
Net unrealized gain (losses) related to assets	0	0	0	0
Total available-for-sale securities | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	194	254	235	306
Total gains (losses) (realized/unrealized) Included in Net Income	1	5	8	(9)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(4)	5	(2)	4
Purchases	0	0	0	268
Sales	0	0	0	(268)
Issues	0	0	0	0
Settlements	(9)	(16)	(59)	(53)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	182	248	182	248
Net unrealized gain (losses) related to assets	0	0	0	0
Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	832	797	861	688
Total gains (losses) (realized/unrealized) Included in Net Income	1	41	30	32
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	4	0	89	0
Sales	(4)	(11)	(48)	(11)
Issues	0	0	0	0
Settlements	(47)	(39)	(147)	(93)
Transfers out of Level 3	(20)	(31)	(72)	(75)
Transfers into Level 3	19	121	72	337
Ending Balance	785	878	785	878
Net unrealized gain (losses) related to assets	3	36	28	5
Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	194	269	333	162
Total gains (losses) (realized/unrealized) Included in Net Income	(62)	380	(187)	470
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	7	(7)	(7)	(8)
Transfers out of Level 3	0	0	0	18
Transfers into Level 3	0	0	0	0
Ending Balance	139	642	139	642
Net unrealized gain (losses) related to assets	(55)	372	(194)	468
Long-term debt
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(452)	(522)	(499)	(473)
Total gains (losses) (realized/unrealized) Included in Net Income	6	(4)	(26)	61
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	2	6	10	36
Transfers out of Level 3	0	1	20	2
Transfers into Level 3	(45)	(1)	(46)	(24)
Ending Balance	(489)	(520)	(489)	(520)
Net unrealized gain (losses) related to liabilities	7	(4)	25	(60)
Long-term debt | Consolidated Trusts
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(55)	(89)	(83)	(75)
Total gains (losses) (realized/unrealized) Included in Net Income	3	(1)	2	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	2	5	10	12
Transfers out of Level 3	0	1	20	2
Transfers into Level 3	(45)	(1)	(46)	(24)
Ending Balance	(101)	(85)	(101)	(85)
Net unrealized gain (losses) related to liabilities	(2)	(1)	(3)	1
Long-term debt | Total senior floating | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(397)	(433)	(416)	(398)
Total gains (losses) (realized/unrealized) Included in Net Income	(9)	(3)	28	(61)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	0	1	0	24
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	(388)	(435)	(388)	(435)
Net unrealized gain (losses) related to liabilities	9	(3)	28	(61)
Other Comprehensive Income (Loss) | Total trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	0	0
Other Comprehensive Income (Loss) | Total trading securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	0	0
Other Comprehensive Income (Loss) | Total trading securities | Other agency
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets		0	0	0
Other Comprehensive Income (Loss) | Total trading securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets		0		0
Other Comprehensive Income (Loss) | Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	(4)	4	(1)	8
Other Comprehensive Income (Loss) | Total available-for-sale securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	1	0
Other Comprehensive Income (Loss) | Total available-for-sale securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	0	0
Other Comprehensive Income (Loss) | Total available-for-sale securities | Mortgage revenue bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	(2)	0	(2)	5
Other Comprehensive Income (Loss) | Total available-for-sale securities | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	(2)	4	0	3
Other Comprehensive Income (Loss) | Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	0	0
Other Comprehensive Income (Loss) | Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to assets	0	0	0	0
Other Comprehensive Income (Loss) | Long-term debt
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to liabilities	0	0	0	0
Other Comprehensive Income (Loss) | Long-term debt | Consolidated Trusts
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to liabilities	0	0	0	0
Other Comprehensive Income (Loss) | Long-term debt | Total senior floating | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to liabilities	$ 0	$ 0	$ 0	$ 0