Short-Term Borrowings and Long-Term Debt Short-Term Borrowings (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Short-Term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase, outstanding	$ 115,000,000	$ 62,000,000
Federal funds purchased and securities sold under agreements to repurchase, weighted-average interest rate	0.00%	0.00%
Short-term debt, outstanding	$ 61,237,000,000	$ 71,950,000,000
Short-term debt, weighted-average interest rate	0.43%	0.26%
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
Line of credit amount outstanding	0
Fannie Mae [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 60,495,000,000	$ 71,007,000,000
Short-term debt, weighted-average interest rate	0.43%	0.26%
Consolidated Trusts [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 742,000,000	$ 943,000,000
Short-term debt, weighted-average interest rate	0.41%	0.19%