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Significant Transactions OneBeacon runoff business (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Significant Transactions |  |  |  |
Gain (loss) on sale of discontinued operation, net of tax | $ (91.0) | $ (18.2) | $ (91.0) | $ (18.2)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest | (15.8) | (12.0) | (24.5) | (7.8)
OneBeacon Runoff [Member] |  |  |  |
Significant Transactions |  |  |  |
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent | (100.5) |  |  |
Gain (loss) on sale of discontinued operation, net of tax | (91.5) |  |  |
Loss attributable to change in discount rate [Member] |  |  |  |
Significant Transactions |  |  |  |
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest | (9.0) |  |  |
Loss attributable to prior year reserve devolopment [Member] |  |  |  |
Significant Transactions |  |  |  |
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest | $ (6.5) |  |  |

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