Short-Term Borrowings (U.S. Government Securities And Related Repurchase Agreements Including Accrued Interest) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
U.S. Government Securities Sold
Short-term Debt [Line Items]
Overnight maturity, Amortized Cost	$ 6,076
Overnight maturity, Fair value	6,120
Repurchase Agreements
Short-term Debt [Line Items]
Overnight maturity, Amortized Cost	$ 5,980
Overnight maturity, Rate	0.016%

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

-Subparagraph 1

-Article 4

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

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