Short-Term Debt (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 33,756,000,000	$ 35,579,000,000
Short-term debt, weighted-average interest rate	1.18%	0.49%
Line of Credit [Member]
Short-Term Debt [Line Items]
Line of credit maximum borrowing capacity	$ 15,000,000,000	$ 15,000,000,000
Consolidated Trusts [Member]
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 379,000,000	$ 584,000,000
Short-term debt, weighted-average interest rate	1.11%	0.48%
Fannie Mae [Member]
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 33,377,000,000	$ 34,995,000,000
Short-term debt, weighted-average interest rate	1.18%	0.49%