Fair Value - Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Total trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 57	$ 95
Total gains (losses) (realized/unrealized) Included in Net Income	(6)	(1)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	(9)	(8)
Transfers into Level 3	5	23
Ending Balance	47	109
Net unrealized gain (losses) related to assets	(4)	2
Total trading securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	57	94
Total gains (losses) (realized/unrealized) Included in Net Income	(6)	(1)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	(9)	(7)
Transfers into Level 3	5	23
Ending Balance	47	109
Net unrealized gain (losses) related to assets	(4)	2
Total trading securities | Other agency
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		1
Total gains (losses) (realized/unrealized) Included in Net Income		0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)		0
Purchases		0
Sales		0
Issues		0
Settlements		0
Transfers out of Level 3		(1)
Transfers into Level 3		0
Ending Balance		0
Net unrealized gain (losses) related to assets		0
Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	753	648
Total gains (losses) (realized/unrealized) Included in Net Income	(7)	3
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(5)	(2)
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(13)	(38)
Transfers out of Level 3	(2)	0
Transfers into Level 3	1	0
Ending Balance	727	611
Net unrealized gain (losses) related to assets	0	0
Total available-for-sale securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	431	195
Total gains (losses) (realized/unrealized) Included in Net Income	1	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(4)	(3)
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(10)	(3)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	418	189
Net unrealized gain (losses) related to assets	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
Total gains (losses) (realized/unrealized) Included in Net Income	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	(1)
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	(2)	0
Transfers into Level 3	0	0
Ending Balance	0	1
Net unrealized gain (losses) related to assets	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	144	216
Total gains (losses) (realized/unrealized) Included in Net Income	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	(1)
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(3)	(24)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	141	191
Net unrealized gain (losses) related to assets	0	0
Total available-for-sale securities | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	176	235
Total gains (losses) (realized/unrealized) Included in Net Income	(8)	3
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(1)	3
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	(11)
Transfers out of Level 3	0	0
Transfers into Level 3	1	0
Ending Balance	168	230
Net unrealized gain (losses) related to assets	0	0
Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	755	861
Total gains (losses) (realized/unrealized) Included in Net Income	(25)	8
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(41)	(47)
Transfers out of Level 3	(44)	(23)
Transfers into Level 3	23	21
Ending Balance	668	820
Net unrealized gain (losses) related to assets	(21)	9
Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	131	333
Total gains (losses) (realized/unrealized) Included in Net Income	(82)	(132)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	8	(35)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	57	166
Net unrealized gain (losses) related to assets	(74)	(167)
Long-term debt
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(468)	(499)
Total gains (losses) (realized/unrealized) Included in Net Income	68	35
Total gains (losses) (realized/unrealized) included in OCI	0	0
Purchases	0	0
Sales	0	0
Issues	(86)	0
Settlements	18	4
Transfers out of Level 3	1	20
Transfers into Level 3	(1)	0
Ending Balance	(468)	(440)
Net unrealized gain (losses) related to liabilities	68	34
Long-term debt | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(373)	(416)
Total gains (losses) (realized/unrealized) Included in Net Income	66	34
Total gains (losses) (realized/unrealized) included in OCI	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	(307)	(382)
Net unrealized gain (losses) related to liabilities	66	34
Long-term debt | Consolidated Trusts
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(95)	(83)
Total gains (losses) (realized/unrealized) Included in Net Income	2	1
Total gains (losses) (realized/unrealized) included in OCI	0	0
Purchases	0	0
Sales	0	0
Issues	(86)	0
Settlements	18	4
Transfers out of Level 3	1	20
Transfers into Level 3	(1)	0
Ending Balance	(161)	(58)
Net unrealized gain (losses) related to liabilities	2	0
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
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
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
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)	0
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	(3)	(2)
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
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	0	0
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	(1)	2
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
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
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
Other Comprehensive Income (Loss) | Long-term debt | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gain (losses) related to liabilities	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