Short-Term and Long-Term Debt Characteristics of Debt (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 3,923,563,000,000	$ 3,440,724,000,000
Earlier of Contractual Maturity or Next Call Date [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,923,563,000,000
Consolidated Trusts [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,646,164,000,000	3,285,139,000,000
Consolidated Trusts [Member] | Earlier of Contractual Maturity or Next Call Date [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,646,164,000,000
Fannie Mae [Member]
Long-Term Debt [Line Items]
Face amount	290,000,000,000	182,200,000,000
Long-Term Debt by Year of Maturity [Abstract]
2021	66,631,000,000
2022	65,593,000,000
2023	30,946,000,000
2024	19,762,000,000
2025	39,593,000,000
Thereafter	54,874,000,000
Total long-term debt	277,399,000,000	155,585,000,000
Net unamortized discount, fair value adjustments and other cost basis adjustments	392,000,000	2,000,000
Fannie Mae [Member] | Zero-Coupon Debt [Member]
Long-Term Debt [Line Items]
Face amount	$ 5,100,000,000	0.0163
Effective interest rate	0.50%
Fannie Mae [Member] | Callable [Member]
Long-Term Debt [Line Items]
Outstanding debt	$ 57,500,000,000	$ 38,500,000,000
Fannie Mae [Member] | Earlier of Contractual Maturity or Next Call Date [Member]
Long-Term Debt by Year of Maturity [Abstract]
2021	92,454,000,000
2022	82,824,000,000
2023	20,814,000,000
2024	14,881,000,000
2025	21,985,000,000
Thereafter	44,441,000,000
Total long-term debt	$ 277,399,000,000