Short-Term and Long-Term Debt - Short-Term Debt (Details) - Fannie Mae - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Short-Term Debt [Line Items]
Short-term debt, weighted-average interest rate (percent)	0.01%	0.18%
Short-term debt, outstanding	$ 2,280	$ 12,173