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Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
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Investment securities |  |
Cost or amortized cost | $ 5,189.6 | $ 6,228.2
Gross unrealized gains | 137.4 | 119.1
Gross unrealized losses | (4.6) | (18.6)
Net foreign currency gains (losses) | (32.7) | 5.0
Total fixed maturity investments including assets held for sale | 5,289.7 | 6,333.7
Fixed maturity investments, at fair value | 4,912.4 | 6,221.9
U.S. Government and agency obligations |  |
Investment securities |  |
Cost or amortized cost | 393.7 | 299.4
Gross unrealized gains | 1.2 | 5.3
Gross unrealized losses | (0.1) | (0.1)
Net foreign currency gains (losses) | (0.1) | 0.4
Fixed maturity investments, at fair value | 394.7 | 305.0
Debt securities issued by corporations |  |
Investment securities |  |
Cost or amortized cost | 2,213.8 | 2,072.1
Gross unrealized gains | 91.6 | 73.7
Gross unrealized losses | (3.0) | (7.8)
Net foreign currency gains (losses) | (21.1) | (2.9)
Fixed maturity investments, at fair value | 2,281.3 | 2,135.1
Municipal obligations |  |
Investment securities |  |
Cost or amortized cost | 3.8 | 2.7
Gross unrealized gains | 0 | 0
Gross unrealized losses | 0 | 0
Net foreign currency gains (losses) | 0 | 0
Fixed maturity investments, at fair value | 3.8 | 2.7
Mortgage-backed and asset-backed securities |  |
Investment securities |  |
Cost or amortized cost | 2,023.3 | 3,190.5
Gross unrealized gains | 30.3 | 25.9
Gross unrealized losses | (1.1) | (3.9)
Net foreign currency gains (losses) | (5.1) | 10.4
Fixed maturity investments, at fair value | 2,047.4 | 3,222.9
Foreign government, agency and provincial obligations |  |
Investment securities |  |
Cost or amortized cost | 475.2 | 581.2
Gross unrealized gains | 8.0 | 11.0
Gross unrealized losses | (0.4) | (0.1)
Net foreign currency gains (losses) | (6.4) | (2.9)
Fixed maturity investments, at fair value | 476.4 | 589.2
Non-cumulative perpetual preference shares |  |
Investment securities |  |
Cost or amortized cost | 79.8 | 82.3
Gross unrealized gains | 6.3 | 3.2
Gross unrealized losses | 0 | (6.7)
Net foreign currency gains (losses) | 0 | 0
Fixed maturity investments, at fair value | 86.1 | 78.8
Common shareholders' equity. |  |
Investment securities |  |
Cost or amortized cost | 870.0 | 691.7
Gross unrealized gains | 139.7 | 72.0
Gross unrealized losses | (4.9) | (8.7)
Net foreign currency gains (losses) | 0 | 0
Convertible fixed maturity investments |  |
Investment securities |  |
Cost or amortized cost | 140.2 | 139.2
Gross unrealized gains | 7.6 | 6.2
Gross unrealized losses | (5.8) | (1.6)
Net foreign currency gains (losses) | 0 | 0
Other long-term investments. |  |
Investment securities |  |
Cost or amortized cost | 264.8 | 274.4
Gross unrealized gains | 57.1 | 55.5
Gross unrealized losses | (8.6) | (25.2)
Net foreign currency gains (losses) | (6.2) | (3.4)
OneBeacon Runoff [Member] |  |
Investment securities |  |
Fixed maturity investments reclassified to assets held for sale |  | 377.3
AutoOne |  |
Investment securities |  |
Fixed maturity investments reclassified to assets held for sale |  | $ 111.8

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- Definition Debt 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 debt 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) -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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- Definition This item represents the cost of trading securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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