Investment Securities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Percentage of debt securities considered to be of investment grade	96.00%
Percentage of tax-exempt securities classified as pre-refunded	14.00%
Percentage of tax-exempt securities insured by bond insurers	25.00%
Residential and commercial mortgage-backed securities, percentage of collateral consisting of prime loans	99.00%
Other-than-temporary impairments	$ 0	$ 0	$ 103.1
Securities Lending Program [Member]
Percentage of collateral to fair value of investment securities on loan	102.00%
Fair value of securities on loan	$ 54.0	$ 126.1

This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.M)

-URI http://asc.fasb.org/extlink&oid=6784337&loc=d3e87990-122713

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration