Short-Term Borrowings (Schedule of Short-term Debt) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Demand Notes	$ 3,481	$ 3,338
Federal Home Loan Bank, Advances, Short-term	800	2,950
Securities Sold under Agreements to Repurchase, Gross	2,166	774
Short-term borrowings	6,447	7,062
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,481	3,338
Federal Home Loan Bank, Advances, Short-term	0	0
Securities Sold under Agreements to Repurchase, Gross	0	0
Short-term borrowings	3,481	3,338
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Federal Home Loan Bank, Advances, Short-term	800	2,950
Securities Sold under Agreements to Repurchase, Gross	2,166	774
Short-term borrowings	2,966	3,724
Secured Debt [Member] | Maturity up to 30 days [Member]
Short-term Debt [Line Items]
Securities Sold under Agreements to Repurchase, Gross	1,700
Secured Debt [Member] | Maturity 31 to 60 days [Member]
Short-term Debt [Line Items]
Securities Sold under Agreements to Repurchase, Gross	$ 504