Investment Securities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Percentage of debt securities considered to be of investment-grade	95.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	12.00%
Percentage of tax-exempt securities insured by bond insurers	22.00%
Residential and commercial mortgage-backed securities, percentage of collateral consisting of prime loans	99.00%
General obligation bonds as percentage of tax exempt municipal securities	45.00%
Other-than-temporary impairments	$ 0	$ 0	$ 0
Special revenue bonds as percentage of tax exempt municipal securities	55.00%
Maximum percentage of any general obligation bonds in one state	11.00%
Percentage of corporate securities associated with financial services industry	19.30%	29.40%
Securities Lending Program [Member]
Percentage of collateral to fair value of investment securities on loan	102.00%
Fair value of securities on loan		$ 54