Short-Term and Long-Term Debt - Characteristics of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 4,216,909	$ 4,205,404
Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	4,216,909
Fannie Mae
Long-Term Debt [Line Items]
Face amount	143,100	128,100
Long-Term Debt by Year of Maturity [Abstract]
2025	38,265
2026	40,249
2027	13,437
2028	5,501
2029	8,350
Thereafter	22,432
Total long-term debt	128,234	106,751
Fannie Mae | Zero-coupon debt
Long-Term Debt [Line Items]
Face amount	$ 11,500	$ 11,600
Effective interest rate	4.36%	5.06%
Fannie Mae | Callable
Long-Term Debt [Line Items]
Outstanding debt	$ 41,000	$ 43,800
Fannie Mae | Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
2025	63,236
2026	38,317
2027	6,859
2028	1,391
2029	2,890
Thereafter	15,541
Total long-term debt	128,234
Consolidated Trusts
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	4,088,675	$ 4,098,653
Consolidated Trusts | Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 4,088,675