Investment Securities (The Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment securities
Common equity securities, at fair value	$ 1,029.5	$ 1,029.7
Convertible fixed maturity investments, at fair value	74.1	127.4
Other long-term investments	303.3	294.2
Common shareholders' equity.
Investment securities
Cost or amortized cost	852.4	895.2
Gross unrealized gains	189.4	143.4
Trading Securities, Gross Unrealized Loss	(13.2)	(8.8)
Net foreign currency gains (losses)	0.9	(0.1)
Common equity securities, at fair value	1,029.5	1,029.7
Convertible Debt Securities
Investment securities
Cost or amortized cost	71.1	121.7
Gross unrealized gains	3.4	6.1
Trading Securities, Gross Unrealized Loss	(0.5)	(0.4)
Net foreign currency gains (losses)	0.1	0
Convertible fixed maturity investments, at fair value	74.1	127.4
Other long-term investments.
Investment securities
Cost or amortized cost	257.7	257.2
Gross unrealized gains	72.6	65.9
Trading Securities, Gross Unrealized Loss	(25.4)	(22.8)
Net foreign currency gains (losses)	(1.6)	(6.1)
Other long-term investments	$ 303.3	$ 294.2

Total common and preferred stock (which is not mandatorily redeemable, redeemable at the option of the holder, or in any other way classified as liability) equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for equity securities formerly categorized as available-for-sale or held-to-maturity which the entity held as of the date it opted to account for such securities at fair value. An enterprise may also categorize such a security as trading without the intent to sell it in the near term assuming the decision to categorize the security as trading occurred at acquisition.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

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-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 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph a

-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 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Implementation Guide (Q and A)

-Number FAS115

-Paragraph 35

-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 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

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-Section S99

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-Subparagraph (SX 210.7-03.(a),1(b))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details