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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, 2019 USD ($) multiplier shares | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) shares
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 | 40,616 | 50,515 | 80,353
Shares paid or expired | (13,715) | (23,186) | (30,838)
New grants | 15,600 | 14,105 | 17,710
Forfeitures | (28) | (818) | (16,710)
End of period | 42,473 | 40,616 | 50,515
Unamortized grant date fair value |  |  |
Beginning of period | $ | $ 31.7 | $ 45.8 | $ 42.4
Shares paid or expired | $ | (18.1) | (28.4) | (21.9)
Forfeitures | $ | (0.1) | 0.1 | (9.3)
Expense recognized | $ | 30.2 | 14.2 | 34.6
End of period | $ | 43.7 | $ 31.7 | $ 45.8
Unamortized grant date fair value | $ | $ 17.2 |  |
W T M Incentive Plan | Sirius Group |  |  |
Target Performance Share activity |  |  |
Beginning of period |  | 2,195 |
End of period |  |  | 2,195
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2016-2018 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout | 139.00% |  |
Payout for maximum growth target percentage | 166.00% |  |
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2015 to 2017 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout |  | 145.00% |
Payout for maximum growth target percentage |  | 147.00% |
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2014 to 2016 |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout |  |  | 34.00%
Payout for maximum growth target percentage |  |  | 76.00%

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- Definition Amount of expense for award under share-based payment arrangement. Excludes amount capitalized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&loc=d3e5070-113901 + Details
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- Definition Amount of cost not yet recognized for nonvested award under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=116856206&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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&loc=d3e5070-113901 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + 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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