Short-Term Borrowings and Long-Term Debt Short-Term Borrowings (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Short-Term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase, outstanding	$ 81,000,000	$ 0
Short-term debt, outstanding	$ 33,791,000,000	$ 35,579,000,000
Short-term debt, weighted-average interest rate	1.02%	0.49%
Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member]
Short-Term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase, weighted-average interest rate	0.25%	0.00%
Intraday line of credit [Member]
Short-Term Borrowings [Line Items]
Line of credit maximum borrowing capacity	$ 15,000,000,000	$ 15,000,000,000
Consolidated Trusts [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 459,000,000	$ 584,000,000
Short-term debt, weighted-average interest rate	0.78%	0.48%
Fannie Mae [Member]
Short-Term Borrowings [Line Items]
Short-term debt, outstanding	$ 33,332,000,000	$ 34,995,000,000
Short-term debt, weighted-average interest rate	1.03%	0.49%