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Employee Share-Based Incentive Compensation Plans (OneBeacon - Non-Qualified Options) (Details) (USD $)
In Millions, except Share data, unless otherwise specified |
1 Months Ended | 3 Months Ended | 9 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Sep. 30, 2012
|
Dec. 31, 2011
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Nov. 30, 2006
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Sep. 30, 2012
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Sep. 30, 2011
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Sep. 30, 2012
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Sep. 30, 2011
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Nov. 16, 2010
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
May 27, 2008
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
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Mar. 31, 2008
Non-Qualified Options
One Beacon Long Term Incentive Plan [Member]
|
Sep. 30, 2012
Grant Amendments [Member]
Non-Qualified Options
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Sep. 30, 2012
Grant Amendments [Member]
OneBeacon
Non-Qualified Options
|
Sep. 30, 2012
Grant Amendments [Member]
OneBeacon
Non-Qualified Options
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| Share-based compensation arrangement by share-based payment award | |||||||||||||
| Granted (in shares) | 1,420,000 | ||||||||||||
| Target Performance Share activity | |||||||||||||
| Opening balance - outstanding Options (in shares) | 0 | 750,130 | 740,870 | 768,652 | |||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | 0 | 0 | 0 | 0 | |||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period | 0 | (9,260) | 0 | (27,782) | |||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Options Vested and Expirations in Period | 0 | 0 | (740,870) | 0 | |||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | 0 | 0 | 0 | 0 | |||||||||
| Ending balance - outstanding Options (in shares) | 0 | 740,870 | 0 | 740,870 | |||||||||
| Unamortized grant date fair value | |||||||||||||
| Accrued expense at the beginning of period | $ 144.7 | $ 187.9 | |||||||||||
| Expense recognized | 0.1 | 0.1 | 0.4 | ||||||||||
| Accrued expense at the end of the period | $ 144.7 | $ 187.9 | |||||||||||
| Expected volatility (as a percent) | 30.00% | ||||||||||||
| Risk-free rate (as a percent) | 4.60% | ||||||||||||
| Forfeiture (as a percent) | 5.00% | ||||||||||||
| Expected dividend yield (as a percent) | 3.40% | ||||||||||||
| Expected life (in years) | 5 years 6 months | ||||||||||||
| Initial exercise price (in dollars per share) | $ 30.00 | ||||||||||||
| Amended exercise price of option (in dollars per share) | $ 25.47 | $ 27.97 | |||||||||||
| OneBeacon special dividend (in dollars per share) | $ 2.50 | $ 2.03 | |||||||||||