Short-Term and Long-Term Debt - Characteristics of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 4,155,396	$ 3,923,563
Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	4,155,396
Fannie Mae
Long-Term Debt [Line Items]
Face amount	202,500	290,000
Long-Term Debt by Year of Maturity [Abstract]
2022	65,617
2023	23,255
2024	18,817
2025	38,141
2026	9,598
Thereafter	42,669
Total long-term debt	198,097	277,399
Fannie Mae | Zero-coupon debt
Long-Term Debt [Line Items]
Face amount	$ 3,200	$ 5,100
Effective interest rate	0.66%	0.50%
Fannie Mae | Callable
Long-Term Debt [Line Items]
Outstanding debt	$ 47,000	$ 57,500
Fannie Mae | Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
2022	100,564
2023	20,856
2024	14,169
2025	21,614
2026	7,195
Thereafter	33,699
Total long-term debt	198,097
Consolidated Trusts
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,957,299	$ 3,646,164
Consolidated Trusts | Assuming Callable Debt Redeemed at Next Available Call Date
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 3,957,299