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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon [Member] - One Beacon Long Term Incentive Plan $ in Millions |  |  | 1 Months Ended | 3 Months Ended |
--- | --- | --- | --- | --- | ---
Feb. 28, 2017 shares | Feb. 24, 2016 shares | May 31, 2011 shares | Mar. 31, 2017 USD ($) shares | Mar. 31, 2016 USD ($) shares | May 25, 2011 Installment
Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 395,872 | 382,722 |
Issued (in shares) | 461,160 | 170,650 |  | 461,160 | 170,650 |
Vested (in shares) |  |  |  | (157,500) | (157,500) |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 |
Ending of period (in shares) |  |  |  | 699,532 | 395,872 |
Unamortized grant date fair value |  |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ |  |  |  | $ 2.1 | $ 2.5 |
Issued | $ |  |  |  | 7.4 | 2.3 |
Assumed forfeitures and cancellations | $ |  |  |  | 0.0 | 0.0 |
Expense recognized | $ |  |  |  | (0.9) | (0.5) |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 8.6 | $ 4.3 |
Restricted Stock | Chief Executive Officer |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) |  |  | 630,000 |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Number of Installments in which the Shares Vest | Installment |  |  |  |  |  | 4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years |  |  | 35,000 |  |  |
Performance Share |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 452,519 | 449,435 |
Assumed forfeitures and cancellations |  |  |  | (13) | (4,079) |
Ending of period (in shares) |  |  |  | 309,796 | 441,206 |
Unamortized grant date fair value |  |  |  |  |  |
Assumed forfeitures and cancellations | $ |  |  |  | $ 0.0 | $ 0.0 |
Expense recognized | $ |  |  |  | (0.3) | $ (0.4) |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 1.8 |  |
Performance Share | Chief Executive Officer |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Annual reduction in performance shares |  |  | 35,000 |  |  |
Years Performance Shares will be Forfeited |  |  | 5 years |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) |  |  |  | 240,160 | 222,449 |
Ending of period (in shares) |  |  |  | 208,499 |  |
Unamortized grant date fair value |  |  |  |  |  |
Expense recognized | $ |  |  |  | $ (0.6) | $ (0.4) |
Restricted Share Cliff Vesting in August 2018 [Member] | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 235,000 |  |  |  |  |
Restricted Share Cliff Vesting in January 2020 [Member] | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 115,450 |  |  |  |  |
Restricted Share Vesting in February 2018 and February 2019 [Member] | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 110,710 |  |  |  |  |
Restricted Share Vesting in February 2018 [Member] | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) |  | 92,500 |  |  |  |
Restricted Share Vesting in February 2019 [Member] | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) |  | 78,150 |  |  |  |

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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 equity-based payment instruments, excluding stock (or unit) options, that vested 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)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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