Short-Term and Long-Term Debt Short-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 26,065	$ 33,756
Short-term debt, weighted-average interest rate	1.86%	1.18%
Intraday line of credit [Member]
Short-Term Debt [Line Items]
Line of credit maximum borrowing capacity	$ 15,000	$ 15,000
Consolidated Trusts [Member]
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 339	$ 379
Short-term debt, weighted-average interest rate	1.88%	1.11%
Fannie Mae [Member]
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 25,726	$ 33,377
Short-term debt, weighted-average interest rate	1.86%	1.18%