Fair Value - Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement Of Income, Extensible List Not Disclosed Flag	Total long-term debt	Total long-term debt	Total long-term debt	Total long-term debt
Fair Value, Recurring Basis, Unobservable Input, Reconciliation, Liability Gain (Loss), Statement Of Income, Extensible List, Not Disclosed Flag	Included in Net Income	Included in Net Income	Included in Net Income	Included in Net Income
Fair Value, Recurring Basis, Unobservable Input, Reconciliation, Asset Gain (Loss), Statement Of Income, Extensible List, Not Disclosed Flag	Included in Net Income	Included in Net Income	Included in Net Income	Included in Net Income
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement Of Income, Extensible List Not Disclosed Flag	Net Unrealized Gains (Losses) Included in Net Income Related to Assets and Liabilities Still Held as of September 30, 2024(4)(5)	Net Unrealized Gains (Losses) Included in Net Income Related to Assets and Liabilities Still Held as of September 30, 2023(4)(5)	Net Unrealized Gains (Losses) Included in Net Income Related to Assets and Liabilities Still Held as of September 30, 2024(4)(5)	Net Unrealized Gains (Losses) Included in Net Income Related to Assets and Liabilities Still Held as of September 30, 2023(4)(5)
Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 461	$ 591	$ 514	$ 634
Total gains (losses) (realized/unrealized) included in net income	0	1	2	7
Total gains (losses) (realized/unrealized) Included in OCI (loss)	6	(19)	7	(13)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(19)	(42)	(74)	(97)
Transfers out of Level 3	0	(117)	(1)	(118)
Transfers into Level 3	0	1	0	2
Ending Balance	448	415	448	415
Net unrealized gain (losses) related to assets	5	(12)	6	(6)
Mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	425	510	477	543
Total gains (losses) (realized/unrealized) included in net income	18	(2)	15	1
Total gains (losses) (realized/unrealized) Included in OCI (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(6)	0
Issues	0	0	0	0
Settlements	(14)	(19)	(47)	(64)
Transfers out of Level 3	(8)	(15)	(47)	(30)
Transfers into Level 3	9	17	38	41
Ending Balance	430	491	430	491
Net unrealized gain (losses) related to assets	9	(4)	8	(4)
Net derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	94	12	77	(37)
Total gains (losses) (realized/unrealized) included in net income	(19)	28	(31)	60
Total gains (losses) (realized/unrealized) Included in OCI (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	5	(1)	34	16
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	80	39	80	39
Net unrealized gain (losses) related to assets	(14)	26	3	76
Long-term debt:
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(380)	(381)	(385)	(378)
Total gains (losses) (realized/unrealized) included in net income	(18)	17	(21)	3
Total gains (losses) (realized/unrealized) included in OCI (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	5	4	12	15
Transfers out of Level 3	0	52	1	52
Transfers into Level 3	(1)	(1)	(1)	(1)
Ending Balance	(394)	(309)	(394)	(309)
Net unrealized gain (losses) related to liabilities	(17)	16	(21)	3
Fannie Mae | Long-term debt:
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(270)	(256)	(268)	(242)
Total gains (losses) (realized/unrealized) included in net income	(14)	16	(16)	2
Total gains (losses) (realized/unrealized) included in OCI (loss)	0	0	0	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	0
Ending Balance	(284)	(240)	(284)	(240)
Net unrealized gain (losses) related to liabilities	(14)	16	(16)	2
Consolidated Trusts | Long-term debt:
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(110)	(125)	(117)	(136)
Total gains (losses) (realized/unrealized) included in net income	(4)	1	(5)	1
Total gains (losses) (realized/unrealized) included in OCI (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	5	4	12	15
Transfers out of Level 3	0	52	1	52
Transfers into Level 3	(1)	(1)	(1)	(1)
Ending Balance	(110)	(69)	(110)	(69)
Net unrealized gain (losses) related to liabilities	(3)	0	(5)	1
Mortgage-related | Total trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	32	28	26	47
Total gains (losses) (realized/unrealized) included in net income	2	(4)	1	(12)
Total gains (losses) (realized/unrealized) Included in OCI (loss)	0	0	0	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	(2)	(4)	(5)	(15)
Transfers into Level 3	3	4	13	4
Ending Balance	35	24	35	24
Net unrealized gain (losses) related to assets	2	(4)	1	(9)
Agency securities | Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	315	356	331	371
Total gains (losses) (realized/unrealized) included in net income	1	0	1	1
Total gains (losses) (realized/unrealized) Included in OCI (loss)	6	(19)	6	(18)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(9)	(8)	(25)	(25)
Transfers out of Level 3	0	(117)	0	(117)
Transfers into Level 3	0	0	0	0
Ending Balance	313	212	313	212
Net unrealized gain (losses) related to assets	4	(12)	5	(10)
Other mortgage-related | Total available-for-sale securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	146	235	183	263
Total gains (losses) (realized/unrealized) included in net income	(1)	1	1	6
Total gains (losses) (realized/unrealized) Included in OCI (loss)	0	0	1	5
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(10)	(34)	(49)	(72)
Transfers out of Level 3	0	0	(1)	(1)
Transfers into Level 3	0	1	0	2
Ending Balance	135	203	135	203
Net unrealized gain (losses) related to assets	$ 1	$ 0	$ 1	$ 4