Fair Value - Recurring Level 3 Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 95	$ 46	$ 33
Total gains or (losses) (realized/unrealized), net income	(24)	(9)	3
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	18	0	77
Sales	0	(95)	(22)
Issues	0	0	0
Settlements	0	(3)	(17)
Transfers out of Level 3	(165)	(49)	(108)
Transfers into Level 3	133	205	80
Ending Balance	57	95	46
Net unrealized gains (losses) included in net income related to assets still held	0	(8)	1
Trading securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	94	45	32
Total gains or (losses) (realized/unrealized), net income	(24)	(12)	3
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	18	0	77
Sales	0	(1)	(22)
Issues	0	0	0
Settlements	0	0	(16)
Transfers out of Level 3	(164)	(48)	(108)
Transfers into Level 3	133	110	79
Ending Balance	57	94	45
Net unrealized gains (losses) included in net income related to assets still held	0	(8)	1
Trading securities | Other agency
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1	1	0
Total gains or (losses) (realized/unrealized), net income	0	0	0
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	0
Settlements	0	0	0
Transfers out of Level 3	(1)	(1)	0
Transfers into Level 3	0	1	1
Ending Balance	0	1	1
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Trading securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	0	1
Total gains or (losses) (realized/unrealized), net income		3	0
Total gains or (losses) (realized/unrealized), OCI		0	0
Purchases		0	0
Sales		(94)	0
Issues		0	0
Settlements		(3)	(1)
Transfers out of Level 3		0	0
Transfers into Level 3		94	0
Ending Balance		0	0
Net unrealized gains (losses) included in net income related to assets still held		0	0
Available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	648	792	952
Total gains or (losses) (realized/unrealized), net income	14	(8)	19
Total gains or (losses) (realized/unrealized), OCI	(7)	5	(11)
Purchases	0	0	0
Sales	0	(1)	(28)
Issues	0	0	0
Settlements	(171)	(177)	(158)
Transfers out of Level 3	(107)	(243)	(106)
Transfers into Level 3	376	280	124
Ending Balance	753	648	792
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Available-for-sale securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	195	171	152
Total gains or (losses) (realized/unrealized), net income	1	1	0
Total gains or (losses) (realized/unrealized), OCI	(1)	4	7
Purchases	0	0	0
Sales	0	(1)	0
Issues	0	0	0
Settlements	(33)	(15)	(8)
Transfers out of Level 3	(107)	(243)	(103)
Transfers into Level 3	376	278	123
Ending Balance	431	195	171
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
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	0	24
Total gains or (losses) (realized/unrealized), net income	0	0	5
Total gains or (losses) (realized/unrealized), OCI	0	0	(5)
Purchases	0	0	0
Sales	0	0	(23)
Issues	0	0	0
Settlements	0	0	(1)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	2	0
Ending Balance	2	2	0
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Available-for-sale securities | Mortgage revenue bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	216	315	434
Total gains or (losses) (realized/unrealized), net income	3	(3)	1
Total gains or (losses) (realized/unrealized), OCI	(5)	2	(3)
Purchases	0	0	0
Sales	0	0	(5)
Issues	0	0	0
Settlements	(70)	(98)	(112)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	144	216	315
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Available-for-sale securities | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	235	306	342
Total gains or (losses) (realized/unrealized), net income	10	(6)	13
Total gains or (losses) (realized/unrealized), OCI	(1)	(1)	(10)
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	0
Settlements	(68)	(64)	(37)
Transfers out of Level 3	0	0	(3)
Transfers into Level 3	0	0	1
Ending Balance	176	235	306
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	861	688	937
Total gains or (losses) (realized/unrealized), net income	31	47	46
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	89	0	0
Sales	(66)	(21)	(52)
Issues	0	0	0
Settlements	(194)	(132)	(136)
Transfers out of Level 3	(86)	(104)	(254)
Transfers into Level 3	120	383	147
Ending Balance	755	861	688
Net unrealized gains (losses) included in net income related to assets still held	26	11	26
Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	333	162	194
Total gains or (losses) (realized/unrealized), net income	(209)	233	109
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	0
Settlements	7	(80)	(119)
Transfers out of Level 3	0	18	(10)
Transfers into Level 3	0	0	(12)
Ending Balance	131	333	162
Net unrealized gains (losses) included in net income related to assets still held	(202)	159	3
Long-term debt
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(499)	(473)	(552)
Total gains (losses) (realized/unrealized), net income	42	(43)	(55)
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	(2)
Settlements	16	41	19
Transfers out of Level 3	20	5	200
Transfers Into Level 3	(47)	(29)	(83)
Ending Balance	(468)	(499)	(473)
Net unrealized gains (losses) included in net income related to liabilities still held	41	(42)	(51)
Long-term debt | Consolidated Trusts
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(83)	(75)	(201)
Total gains (losses) (realized/unrealized), net income	(1)	(2)	(8)
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	(2)
Settlements	16	18	19
Transfers out of Level 3	20	5	200
Transfers Into Level 3	(47)	(29)	(83)
Ending Balance	(95)	(83)	(75)
Net unrealized gains (losses) included in net income related to liabilities still held	(2)	(1)	(4)
Long-term debt | Senior floating | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(416)	(398)	(351)
Total gains (losses) (realized/unrealized), net income	43	(41)	(47)
Total gains or (losses) (realized/unrealized), OCI	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	0
Settlements	0	23	0
Transfers out of Level 3	0	0	0
Transfers Into Level 3	0	0	0
Ending Balance	(373)	(416)	(398)
Net unrealized gains (losses) included in net income related to liabilities still held	43	(41)	(47)
Other Comprehensive Income (Loss) | Trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Other Comprehensive Income (Loss) | Trading securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Other Comprehensive Income (Loss) | Trading securities | Other agency
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Other Comprehensive Income (Loss) | Trading securities | Private-label and other mortgage securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held		0	0
Other Comprehensive Income (Loss) | Available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	1	4	(3)
Other Comprehensive Income (Loss) | Available-for-sale securities | Fannie Mae
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	2	0	6
Other Comprehensive Income (Loss) | 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 gains (losses) included in net income related to assets still held	1	0	0
Other Comprehensive Income (Loss) | Available-for-sale securities | Mortgage revenue bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	(3)	4	(1)
Other Comprehensive Income (Loss) | Available-for-sale securities | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	1	0	(8)
Other Comprehensive Income (Loss) | Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	0	0	0
Other Comprehensive Income (Loss) | Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to assets still held	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 gains (losses) included in net income related to liabilities still held	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 gains (losses) included in net income related to liabilities still held	0	0	0
Other Comprehensive Income (Loss) | Long-term debt | Senior floating | Fannie Mae
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized gains (losses) included in net income related to liabilities still held	$ 0	$ 0	$ 0