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Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2010
Investment securities |  |  |
Cost or amortized cost |  | $ 5,576.3 |
Common equity securities, at fair value | 755.0 | 1,029.7 |
Convertible fixed maturity investments, at fair value | 143.8 | 127.4 |
Other long-term investments | 301.3 | 294.2 |
Proceeds from sale of maturities | 5,034.0 | 6,997.5 | 4,492.6
Common shareholders’ equity |  |  |
Investment securities |  |  |
Cost or amortized cost | 691.7 | 895.2 |
Gross unrealized gains | 72.0 | 143.4 |
Gross unrealized losses | (8.7) | (8.8) |
Net foreign currency losses | 0 | (0.1) |
Common equity securities, at fair value | 755.0 | 1,029.7 |
Convertible fixed maturity investments |  |  |
Investment securities |  |  |
Cost or amortized cost | 139.2 | 121.7 |
Gross unrealized gains | 6.2 | 6.1 |
Gross unrealized losses | (1.6) | (0.4) |
Net foreign currency losses | 0 | 0 |
Convertible fixed maturity investments, at fair value | 143.8 | 127.4 |
Other long-term investments |  |  |
Investment securities |  |  |
Cost or amortized cost | 274.4 | 257.2 |
Gross unrealized gains | 55.5 | 65.9 |
Gross unrealized losses | (25.2) | (22.8) |
Net foreign currency losses | (3.4) | (6.1) |
Other long-term investments | $ 301.3 | $ 294.2 |

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- Definition The cash inflow from sales, maturities, prepayments, calls and collections of all investments, including securities and other assets, having ready marketability and intended by management to be liquidated, if necessary, beyond the current operating cycle. + References No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),1(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: 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 6: 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 7: 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 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 159 -Paragraph 29 -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 9: 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. + Details
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