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Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 3.9 |  | $ 0.0 | $ 18.0 |
Expense Amortized |  |  |  |  |  |
Accrued expense at the beginning of period |  |  |  | 140.7 |  |
Accrued expense at the end of the period | $ 124.2 |  |  | $ 124.2 |  | $ 140.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Vesting Period |  |  |  | 3 years |  |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015 |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  | 24.30% |  |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member] |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  |  |  | 45.70%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share |  |  |  |  |  |
Target Performance Shares Outstanding |  |  |  |  |  |
Beginning of period (in shares) | 441,206 | 462,147 | 517,470 | 449,435 | 517,470 | 517,470
Shares paid or expired (in shares) | 0 | 0 |  | (167,300) | (181,290) |
New grants (in shares) | 0 | 0 |  | 163,150 | 154,887 |
Assumed forfeitures and cancellations | 0 | 0 |  | (4,079) | (28,920) |
Ending of period (in shares) | 441,206 | 462,147 | 462,147 | 441,206 | 462,147 | 449,435
Expense Amortized |  |  |  |  |  |
Accrued expense at the beginning of period | $ 1.0 | $ 2.4 | $ 3.4 | $ 1.4 | $ 3.4 | $ 3.4
Payments and deferrals | 0.0 | 0.0 |  | (0.7) | (1.5) |
Assumed forfeitures and cancellations | 0.0 | 0.0 |  | 0.0 | 0.0 |
Expense recognized | 0.3 | (0.3) |  | 0.6 | 0.2 |
Accrued expense at the end of the period | 1.3 | $ 2.1 | $ 2.4 | 1.3 | $ 2.1 | $ 1.4
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ 2.3 |  |  | $ 2.3 |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + 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.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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- Definition Period 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, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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