Short-Term Borrowings and Long-Term Debt Short-Term Debt (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Short-Term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase, outstanding	$ 118,000,000	$ 50,000,000
Federal funds purchased and securities sold under agreements to repurchase, weighted-average interest rate	0.00%	0.00%
Short-term debt, outstanding	$ 96,818,000,000	$ 106,572,000,000
Short-term debt, weighted-average interest rate	0.20%	0.11%
Secured uncommitted line of credit [Member]
Short-Term Borrowings [Line Items]
Line of credit maximum borrowing capacity	$ 15,000,000,000	$ 15,000,000,000.0
Line of credit amount outstanding	0
Fannie Mae [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 95,427,000,000	$ 105,012,000,000
Short-term debt, weighted-average interest rate	0.21%	0.11%
Consolidated Trusts [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 1,391,000,000	$ 1,560,000,000
Short-term debt, weighted-average interest rate	0.15%	0.09%