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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) multiplier shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares
Performance Cycle From 2013 to 2015 |  |  |
Unamortized grant date fair value |  |  |
Vested (in shares) |  | 5,000 |
White Mountains |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Multiplier for determining payout, low end of range | multiplier | 0 |  |
Multiplier for determining payout, high end of range | multiplier | 2 |  |
Performance cycle period | 3 years |  |
W T M Incentive Plan | White Mountains |  |  |
Target Performance Share activity |  |  |
Beginning of period | 80,353 | 93,654 | 111,257
Shares paid or expired | (30,838) | (36,294) | (42,959)
New grants | 17,710 | 22,615 | 29,195
Assumed forfeitures and cancellations | (16,710) | 378 | (3,839)
End of period | 50,515 | 80,353 | 93,654
Unamortized grant date fair value |  |  |
Beginning of period | $ | $ 42.4 | $ 57.7 | $ 44.4
Shares paid or expired | $ | (21.9) | (41.0) | (30.8)
Assumed forfeitures and cancellations | $ | (9.3) | 0.5 | (0.3)
Expense recognized | $ | 34.6 | 25.2 | 44.4
End of period | $ | 45.8 | $ 42.4 | $ 57.7
Unamortized grant date fair value | $ | $ 20.2 |  |
W T M Incentive Plan | Sirius Group |  |  |
Target Performance Share activity |  |  |
Beginning of period | 7,315 | 10,826 |
End of period | 2,195 | 7,315 | 10,826
W T M Incentive and Phantom Plan | Performance Cycle From 2013 to 2015 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout | 34.00% |  |
Payout for maximum growth target percentage | 76.00% |  |
W T M Incentive and Phantom Plan | Performance Cycle From 2012 to 2014 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout |  | 140.00% |
Payout for maximum growth target percentage |  | 142.00% |
W T M Incentive and Phantom Plan | Performance Cycle from 2011 to 2013 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout |  |  | 91.00%
Payout for maximum growth target percentage |  |  | 145.50%

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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=96867065&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=96867065&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=96867065&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=96867065&loc=d3e5070-113901 + 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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