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Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012
Investment securities |  |  |  |
Fixed maturity investments, at fair value | $ 5,030.5 |  | $ 5,196.2 |
Including Short Term Fixed |  |  |  |
Investment securities |  |  |  |
Duration of Fixed Maturities |  |  | 2 years 1 month 6 days |
US Government and agency obligations |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 365.5 |  | 440.4 |
Net foreign currency losses | (2.5) |  | (1.2) |
Fixed maturity investments, at fair value | 362.5 |  | 440.1 |
Trading Securities, Unrealized Holding Gain | 0.5 |  | 1.0 |
Trading Securities, Unrealized Holding Loss | (1.0) |  | (0.1) |
Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 2,330.7 |  | 2,321.4 |
Net foreign currency losses | (14.3) |  | (23.0) |
Fixed maturity investments, at fair value | 2,347.2 |  | 2,385.1 |
Trading Securities, Unrealized Holding Gain | 44.0 |  | 88.3 |
Trading Securities, Unrealized Holding Loss | (13.2) |  | (1.6) |
Municipal obligations |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 18.3 |  | 5.3 |
Net foreign currency losses | 0 |  | 0 |
Fixed maturity investments, at fair value | 17.9 |  | 5.2 |
Trading Securities, Unrealized Holding Gain | 0 |  | 0 |
Trading Securities, Unrealized Holding Loss | (0.4) |  | (0.1) |
Mortgage-backed and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 2,027.3 |  | 2,081.0 |
Net foreign currency losses | (5.3) |  | (9.4) |
Fixed maturity investments, at fair value | 2,014.5 |  | 2,095.6 |
Trading Securities, Unrealized Holding Gain | 2.4 |  | 25.1 |
Trading Securities, Unrealized Holding Loss | (9.9) |  | (1.1) |
Foreign government, agency and provincial obligations |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 444.2 |  | 526.6 |
Net foreign currency losses | (4.8) |  | (8.6) |
Fixed maturity investments, at fair value | 439.9 |  | 521.9 |
Trading Securities, Unrealized Holding Gain | 3.7 |  | 6.9 |
Trading Securities, Unrealized Holding Loss | (3.2) |  | (3.0) |
Preferred stocks |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 79.9 |  | 79.9 |
Net foreign currency losses | (0.2) |  | (0.2) |
Fixed maturity investments, at fair value | 84.8 |  | 86.4 |
Trading Securities, Unrealized Holding Gain | 5.1 |  | 6.7 |
Trading Securities, Unrealized Holding Loss | 0 |  | 0 |
Fixed Income Investments [Member] |  |  |  |
Investment securities |  |  |  |
Trading Securities, Cost | 5,265.9 |  | 5,454.6 |
Net foreign currency losses | (27.1) |  | (42.4) |
Total fixed maturity investments including assets held for sale | 5,266.8 |  | 5,534.3 |
Trading Securities, Unrealized Holding Gain | 55.7 |  | 128.0 |
Trading Securities, Unrealized Holding Loss | (27.7) |  | (5.9) |
Excluding Short Term Investments |  |  |  |
Investment securities |  |  |  |
Duration of Fixed Maturities |  |  | 2 years 4 months 24 days |
OneBeacon Runoff |  |  |  |
Investment securities |  |  |  |
Fixed maturity investments reclassified to assets held for sale (1) | $ (236.3) | [1] | $ (338.1) | [1]
[1] | Assets held for sale related to discontinued operations. See Note 21.

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- Definition Amount of investments in debt securities classified as trading. + References Reference 1: 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 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 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 4: 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 5: 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 + Details
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- Definition Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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