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Significant Transactions (OneBeacon Personal Lines and Commercial Lines Sales) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Jul. 31, 2010 One Beacon Personal Lines company | Jun. 30, 2011 One Beacon Personal Lines | Dec. 31, 2010 One Beacon Personal Lines | Dec. 31, 2009 One Beacon Personal Lines | Dec. 31, 2009 One Beacon Commercial Lines | Dec. 31, 2012 One Beacon Commercial Lines | Dec. 31, 2011 One Beacon Commercial Lines | Dec. 31, 2009 One Beacon Commercial Lines
Significant Transactions |  |  |  |  |  |  |  |  |  |  |
Number of insurance companies sold |  |  |  | 2 |  |  |  |  |  |  |
Written percentage |  |  |  | 100.00% |  |  |  |  |  |  |
Assumed risk percentage |  |  |  | 100.00% |  |  |  |  |  |  |
Number of Attorneys in Fact |  |  |  | 2 |  |  |  |  |  |  |
Period to provide certain services |  |  |  | 3 years |  |  |  |  |  |  |
Sales of unconsolidated affiliates, net of cash sold and held in escrow | $ 24.8 | $ 1,010.6 | $ 188.3 | $ 166.6 |  |  |  | $ 23.2 | $ 0.8 | $ 10.2 |
Gain including tax benefit |  |  |  |  |  | 24.6 |  |  |  |  |
Gain on disposal |  |  |  |  |  | 8.5 |  |  |  |  |
Tax effect of income (loss) on disposal |  |  |  |  | 5.6 | 16.1 |  |  |  |  |
Premiums written, net | 2,126.9 | 1,978.4 | 2,033.5 |  |  |  | 420.0 |  |  |  |
Value of renewal rights in premiums |  |  |  |  |  |  |  |  | $ 200.0 |  | $ 490.0

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