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Significant Transactions (Details) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |  |  |  |  | 3 Months Ended |  | 3 Months Ended | 2 Months Ended | 1 Months Ended |  |  |  | 3 Months Ended | 9 Months Ended | 3 Months Ended | 2 Months Ended | 3 Months Ended | 2 Months Ended |  |
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Sep. 30, 2014 USD ($) | Sep. 30, 2013 USD ($) | Sep. 30, 2014 USD ($) | Sep. 30, 2013 USD ($) | Dec. 31, 2013 USD ($) | Jul. 01, 2014 durchblicker.at [Member] | Sep. 30, 2014 Star & Shield Insurance Exchange USD ($) | Jan. 31, 2013 Star & Shield Insurance Exchange USD ($) | Mar. 31, 2014 QuoteLab USD ($) | Mar. 14, 2014 QuoteLab USD ($) | Mar. 31, 2013 OneBeacon Essentia USD ($) | Mar. 31, 2014 WM Solutions [Member] Citation [Member] USD ($) | Mar. 31, 2013 WM Solutions [Member] Ashmere USD ($) | Feb. 19, 2014 WOBI business | Jan. 31, 2014 Star & Shield LLC [Member] USD ($) | Sep. 30, 2014 Common Class A QuoteLab | Mar. 14, 2014 Common Class A QuoteLab | Feb. 19, 2014 Common Stock WOBI | Sep. 30, 2013 Non-controlling interest Star & Shield Insurance Exchange USD ($) | Sep. 30, 2014 Non-controlling interest Star & Shield Insurance Exchange USD ($) | Sep. 30, 2014 Euro Member Countries, Euro Common Stock durchblicker.at [Member] ILS | Feb. 19, 2014 Israel, New Shekels Preferred Stock WOBI ILS | Feb. 19, 2014 Israel, New Shekels Common Stock WOBI ILS | Feb. 19, 2014 United States of America, Dollars Preferred Stock WOBI USD ($) | Sep. 30, 2014 United States of America, Dollars Common Stock durchblicker.at [Member] ILS | Feb. 19, 2014 United States of America, Dollars Common Stock WOBI USD ($) | Feb. 19, 2014 WOBI USD ($) | Jun. 30, 2014 PassportCard [Member] USD ($) | Jun. 30, 2014 PassportCard [Member] White Mountains | Jun. 30, 2014 PassportCard [Member] DavidShield [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownerhsip Percentage of Common Equity |  |  |  |  |  | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 21.0 |  |
Share of joint venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50 | 50
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.70% |  | 58.40% | 60.00% | 54.00% |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  | 32.2 | 9.2 |  |  |  |  | 28.1 |  |  |  |  |  | 1.8 |  |  |  |  |  | 8.5 | 12.7 | 14.4 | 3.6 | 11.7 | 4.1 |  |  |  |
Premiums Written, Percentage of Total |  |  |  |  |  |  |  |  |  |  |  |  |  | 85.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of lines of business |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of Surplus Notes |  |  |  |  |  |  |  | 17.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of gross profit |  |  |  |  |  |  |  |  |  | 62.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets | 12,188.3 |  | 12,188.3 |  | 12,144.3 |  | 11.8 |  |  | 56.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13.4 |  |  |
Intangible Assets, Net (Including Goodwill) |  |  |  |  |  |  |  |  |  | 43.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible Assets, Net |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.4 |  |  |
Liabilities | 7,689.5 |  | 7,689.5 |  | 7,746.9 |  | 22.8 |  |  | 10.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.7 |  |  |
Net Income (Loss) Attributable to Noncontrolling Interest | 11.2 | 1.1 | 7.1 | 12.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.4 | 11.1 |  |  |  |  |  |  |  |  |  |
Gain on sale of subsidiary |  |  | 0.7 | 23.0 |  |  |  |  |  |  | 23.0 | 0.7 | 6.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of subsidiary, after tax |  |  |  |  |  |  |  |  |  |  | $ 15.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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