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Employee Share-Based Incentive Compensation Plans (Details 5) (USD $) In Millions, except Share data, unless otherwise specified |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended
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Jun. 30, 2011 | Dec. 31, 2010 | Nov. 30, 2006 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Jun. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Jun. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Nov. 16, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | May 27, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options | May 31, 2010 Non-Qualified Options | Jun. 30, 2010 Non-Qualified Options | Jun. 30, 2011 Non-Qualified Options | Jun. 30, 2010 Non-Qualified Options
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |
Granted (in shares) |  |  | 1,420,000 |  |  |  |  |  |  |  |  |  |  |
Target options outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  |
Opening balance - outstanding Options (in shares) |  |  |  | 768,652 | 916,827 | 768,652 | 1,015,610 |  |  |  |  |  |  |
Forfeitures and cancellations (in shares) |  |  |  | (18,522) | (37,044) | (18,522) | (37,044) |  |  |  |  |  |  |
Vested and expired (in shares) |  |  |  |  |  |  | (98,783) |  |  |  |  |  |  |
Ending balance - outstanding Options (in shares) |  |  |  | 750,130 | 879,783 | 750,130 | 879,783 |  |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued expense at the beginning of period | $ 153.3 | $ 157.4 |  | $ 4.6 | $ 4.0 | $ 4.5 | $ 3.6 |  |  |  |  |  |  |
Expense recognized |  |  |  | 0.2 | 0.1 | 0.3 | 0.5 |  |  |  | 8.7 | 9.7 | 0.1 | 11.1
Accrued expense at the end of the period | $ 153.3 | $ 157.4 |  | $ 4.8 | $ 4.1 | $ 4.8 | $ 4.1 |  |  |  |  |  |  |
Options vesting period (in years) |  |  |  |  |  | Third, fourth and fifth anniversaries |  |  |  |  |  |  |  |
Expiration period of award (in years) |  |  |  |  |  |  | 5.5 |  |  |  |  |  |  |
Assumptions of stock-based compensation awards |  |  |  |  |  |  |  |  |  |  |  |  |  |
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) |  |  |  |  |  | 5.5 |  |  |  |  |  |  |  |
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 |  |  |  |

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- Definition The date when the equity-based award expires as specified in the award agreement, which may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Description of the period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, which may be expressed in a variety of ways (for example, in years, month and year). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64, 65 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph e(2)(c) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.D.2) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph e(2)(a) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph e(2)(b) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph e(2)(d) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(f) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(d) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(a) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(b) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + Details
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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + Details
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- Definition Special dividend per common share paid by OneBeacon. + Details
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- Definition The rate of forfeitures that is used in valuing an option. + Details
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- Definition This element represents the initial exercise price per option to purchase common shares. + Details
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- Definition This element represents the amended exercise price per OneBeacon non-qualified option. The exercise price was amended as a result of the special dividend(s) paid on OneBeacon common shares. + Details
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- Definition This element represents OneBeacon stock options that vested as a result of the Commercial Lines Transaction and the Personal Lines Transaction. The OneBeacon options were not exercised (as they were out-of-the-money) and subsequently expired. + Details
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