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Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Expense Amortized |  |  |
Beginning of period | $ 140.7 |  |
End of period (3) | $ 140.0 | $ 140.7 |
OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |
Target options outstanding |  |  |
Beginning of period | 449,435 | 517,470 | 493,421
Shares paid or expired (in shares) | (167,300) | (181,290) | (142,138)
New grants | 163,150 | 154,887 | 165,800
Assumed forfeitures and cancellations (in shares) | 7,234 | (41,632) | 387
End of period (3) | 452,519 | 449,435 | 517,470
Expense Amortized |  |  |
Beginning of period | $ 1.4 | $ 3.4 | $ 4.0
Payments and deferrals | (0.6) | (1.5) | (1.0)
Assumed forfeitures and cancellations | 0.0 | (0.1) | 0.0
Expense recognized | 0.8 | (0.4) | 0.4
End of period (3) | $ 1.6 | $ 1.4 | $ 3.4
Range Of Performance Cycle From 2013 to 2015 [Member] | Long Term Incentive Plan |  |  |
Expense Amortized |  |  |
Percentage of payout for minimum growth target percentage | 24.30% |  |
Performance Cycle From 2012 to 2014 | Long Term Incentive Plan |  |  |
Expense Amortized |  |  |
Percentage of payout for minimum growth target percentage |  | 45.70% |
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan |  |  |
Expense Amortized |  |  |
Percentage of payout for minimum growth target percentage |  |  | 37.10%

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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 + 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 total reduction to unamortized compensation cost for performance shares forfeited during the period. + References No definition available. + Details
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- Definition The amount of 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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- 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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