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Investment Securities (Details 4) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Investment securities |  |  |
Cost or amortized cost | $ 6,228.2 | $ 5,755.9 |
Gross unrealized gains | 119.1 | 133.6 |
Gross unrealized losses | (18.6) | (44.2) |
Net foreign currency gains (losses) | 5.0 | (59.2) |
Total fixed maturity investments including assets held for sale | 6,333.7 |  |
Fixed maturity investments, at fair value | 6,221.9 | 5,786.1 |
Cost or amortized cost |  |  |
Total | 6,228.2 | 5,755.9 |
Carrying value |  |  |
Carrying value, common equity securities | 755.0 | 667.0 |
Carrying value, convertible fixed securities | 143.8 | 143.0 |
Carrying value, other long-term investments | 301.3 | 372.1 |
Sales and maturities of investments, excluding short-term investments | 5,044.5 | 4,492.6 | 3,748.5
AutoOne |  |  |
Investment securities |  |  |
Fixed maturity investments reclassified to assets held for sale | (111.8) |  |
U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Cost or amortized cost | 299.4 | 385.9 |
Gross unrealized gains | 5.3 | 13.6 |
Gross unrealized losses | (0.1) | (0.8) |
Net foreign currency gains (losses) | 0.4 |  |
Fixed maturity investments, at fair value | 305.0 | 398.7 |
Cost or amortized cost |  |  |
Total | 299.4 | 385.9 |
Debt securities issued by corporations |  |  |
Investment securities |  |  |
Cost or amortized cost | 2,072.1 | 2,149.2 |
Gross unrealized gains | 73.7 | 87.9 |
Gross unrealized losses | (7.8) | (21.1) |
Net foreign currency gains (losses) | (2.9) | (37.9) |
Fixed maturity investments, at fair value | 2,135.1 | 2,178.1 |
Cost or amortized cost |  |  |
Total | 2,072.1 | 2,149.2 |
Municipal obligations |  |  |
Investment securities |  |  |
Cost or amortized cost | 2.7 | 3.3 |
Gross unrealized gains |  | 0.1 |
Gross unrealized losses |  | (0.1) |
Fixed maturity investments, at fair value | 2.7 | 3.3 |
Cost or amortized cost |  |  |
Total | 2.7 | 3.3 |
Mortgage-backed and asset-backed securities |  |  |
Investment securities |  |  |
Cost or amortized cost | 3,190.5 | 2,082.0 |
Gross unrealized gains | 25.9 | 18.2 |
Gross unrealized losses | (3.9) | (15.6) |
Net foreign currency gains (losses) | 10.4 | (12.9) |
Fixed maturity investments, at fair value | 3,222.9 | 2,071.7 |
Cost or amortized cost |  |  |
Securities not having single maturity date | 3,190.5 |  |
Total | 3,190.5 | 2,082.0 |
Carrying value |  |  |
Securities not having single maturity date | 3,222.9 |  |
Foreign government, agency and provincial obligations |  |  |
Investment securities |  |  |
Cost or amortized cost | 581.2 | 1,053.6 |
Gross unrealized gains | 11.0 | 7.7 |
Gross unrealized losses | (0.1) | (6.6) |
Net foreign currency gains (losses) | (2.9) | (8.3) |
Fixed maturity investments, at fair value | 589.2 | 1,046.4 |
Cost or amortized cost |  |  |
Total | 581.2 | 1,053.6 |
Non-cumulative perpetual preference shares |  |  |
Investment securities |  |  |
Cost or amortized cost | 82.3 | 81.9 |
Gross unrealized gains | 3.2 | 6.1 |
Gross unrealized losses | (6.7) |  |
Net foreign currency gains (losses) |  | (0.1) |
Fixed maturity investments, at fair value | 78.8 | 87.9 |
Cost or amortized cost |  |  |
Securities not having single maturity date | 82.3 |  |
Total | 82.3 | 81.9 |
Carrying value |  |  |
Securities not having single maturity date | 78.8 |  |
Fixed maturity investments and convertible fixed maturity investments, excluding mortgage-backed securities, asset-backed securities and preferred stock |  |  |
Cost or amortized cost |  |  |
Due in one year or less | 669.6 |  |
Due after one year through five years | 1,809.1 |  |
Due after five years through ten years | 563.1 |  |
Due after ten years | 52.8 |  |
Carrying value |  |  |
Due in one year or less | 681.0 |  |
Due after one year through five years | 1,857.5 |  |
Due after five years through ten years | 582.8 |  |
Due after ten years | 54.5 |  |
Fixed maturity investments and convertible fixed maturity investments |  |  |
Investment securities |  |  |
Cost or amortized cost | 6,367.4 |  |
Cost or amortized cost |  |  |
Total | 6,367.4 |  |
Carrying value |  |  |
Total | 6,477.5 |  |
Common shareholders' equity. |  |  |
Investment securities |  |  |
Cost or amortized cost | 691.7 | 561.2 |
Gross unrealized gains | 72.0 | 105.5 |
Gross unrealized losses | (8.7) | (1.5) |
Net foreign currency gains (losses) |  | 1.8 |
Cost or amortized cost |  |  |
Total | 691.7 | 561.2 |
Convertible fixed maturities |  |  |
Investment securities |  |  |
Cost or amortized cost | 139.2 | 126.9 |
Gross unrealized gains | 6.2 | 16.2 |
Gross unrealized losses | (1.6) | (0.1) |
Cost or amortized cost |  |  |
Total | 139.2 | 126.9 |
Other long-term investments. |  |  |
Investment securities |  |  |
Cost or amortized cost | 274.4 | 329.5 |
Gross unrealized gains | 55.5 | 62.4 |
Gross unrealized losses | (25.2) | (13.0) |
Net foreign currency gains (losses) | (3.4) | (6.8) |
Cost or amortized cost |  |  |
Total | $ 274.4 | $ 329.5 |

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- Definition The total of financial instruments 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 debt and 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 -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: 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 3: 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 4: 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 5: 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 6: 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 7: 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 8: 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 + Details
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