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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - OneBeacon [Member] - USD ($) $ in Millions |  |  | 3 Months Ended |  |
--- | --- | --- | --- | --- | ---
Feb. 28, 2017 | Feb. 24, 2016 | Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Restricted Stock Units (RSUs) [Member] | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Issued (in shares) |  |  | 240,160 | 222,449 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number |  |  | 208,499 |  |  |
Expense recognized |  |  | $ 0.6 | $ 0.4 |  |
Restricted Stock | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Issued (in shares) | 461,160 | 170,650 | 461,160 | 170,650 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number |  |  | 699,532 | 395,872 | 395,872 | 382,722
Expense recognized |  |  | $ 0.9 | $ 0.5 |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  | 8.6 | $ 4.3 | $ 2.1 | $ 2.5
Scenario, Plan | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  | $ 7.0 |  |  |

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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 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 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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