Fair Value - Fair Value Option (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans	$ 3,315	$ 3,645
Fair value of nonaccrual loans	$ 32	40
Serious delinquency: days past due	90 days
Fair value of loans that are 90 days past due	$ 31	48
Changes in fair value, gain (loss)	1,304	1,284	$ 155
Fannie Mae
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt	761	1,161
Consolidated Trusts
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt	14,343	16,260
Loans
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans	3,315	3,645
Loans, unpaid principal balance	3,442	3,835
Changes in fair value, gain (loss)	108	(503)	28
Long-term debt:
Fair Value Inputs, Quantitative Information [Line Items]
Changes in fair value, gain (loss)	(308)	2,300	$ 631
Long-term debt: | Fannie Mae
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt	761	1,161
Debt, unpaid principal balance	731	1,145
Long-term debt: | Consolidated Trusts
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt	14,343	16,260
Debt, unpaid principal balance	$ 14,383	$ 16,311