Short-Term and Long-Term Debt Characteristics of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 3,367,024	$ 3,296,298
Consolidated Trusts [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,159,846	3,052,923
Earlier of Contractual Maturity or Next Call Date [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,367,024
Earlier of Contractual Maturity or Next Call Date [Member] | Consolidated Trusts [Member]
Long-Term Debt by Year of Maturity [Abstract]
Total long-term debt	3,159,846
Fannie Mae [Member]
Long-Term Debt [Line Items]
Face amount	232,500	277,500
Long-Term Debt by Year of Maturity [Abstract]
2019	61,622
2020	44,348
2021	29,903
2022	8,571
2023	5,267
Thereafter	57,467
Total long-term debt	207,178	243,375
Net unamortized discount, fair value adjustments and other cost basis adjustments	413	752
Fannie Mae [Member] | Zero-Coupon Debt [Member]
Long-Term Debt [Line Items]
Face amount	$ 23,200	$ 39,800
Effective interest rate	4.15%	2.29%
Fannie Mae [Member] | Callable [Member]
Long-Term Debt [Line Items]
Outstanding debt	$ 64,300	$ 72,300
Fannie Mae [Member] | Earlier of Contractual Maturity or Next Call Date [Member]
Long-Term Debt by Year of Maturity [Abstract]
2019	91,202
2020	31,275
2021	18,987
2022	7,487
2023	4,522
Thereafter	53,705
Total long-term debt	$ 207,178