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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions | Total | Common Shares and Paid-in Surplus | Retained Earnings | AOCI, after tax | Parent | Non-controlling Interests | Total Equity | One Beacon Insurance Company [Member] Non-controlling Interests | One Beacon Insurance Company [Member] Total Equity | Star & Shield LLC [Member] Non-controlling Interests | Star & Shield LLC [Member] Total Equity | BAM Non-controlling Interests | BAM Total Equity | Foreign Currency Gain (Loss) AOCI, after tax | Foreign Currency Gain (Loss) Parent | Foreign Currency Gain (Loss) Non-controlling Interests | Foreign Currency Gain (Loss) Total Equity
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Beginning balance at Dec. 31, 2016 |  | $ 810.7 | $ 2,776.6 | $ (4.6) | $ 3,582.7 | $ 133.3 | $ 3,716.0 |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) | $ 593.1 |  | 627.2 |  | 627.2 | (34.1) | 593.1 |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and interest rate swap |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.4 | $ 0.4 | $ 0.1 | $ 0.5
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.9 | $ 2.9 |  | $ 2.9
Comprehensive income (loss) | 630.7 |  | 627.2 | 3.3 | 630.5 | (34.0) | 596.5 |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (4.6) |  | (4.6) |  | (4.6) |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  |  |  |  |  | (19.3) | (19.3) |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 1.7 |  |  | 1.7 |  | 1.7 |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (723.9) | (147.9) | (576.0) |  | (723.9) | (5.2) | (729.1) |  |  |  |  |  |  |  |  |  |
Capital contributions from non-controlling interests |  | (4.6) |  |  | (4.6) | 3.0 | (1.6) |  |  |  |  | $ 27.2 | $ 27.2 |  |  |  |
Issuance of shares of non-controlling interests |  | (4.1) |  |  | (4.1) | 5.2 | 1.1 |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 14.8 |  |  | 14.8 | 0.8 | 15.6 |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests |  | 0.0 |  |  |  |  |  | $ (238.3) | $ (238.3) | $ (4.4) | $ (4.4) |  |  |  |  |  |
Ending balance at Dec. 31, 2017 |  | 670.6 | 2,823.2 | (1.3) | 3,492.5 | (131.7) | 3,360.8 |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) | (191.4) |  | (141.2) |  | (141.2) | (50.2) | (191.4) |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and interest rate swap |  |  |  | (4.5) | (4.5) | (0.3) | (4.8) |  |  |  |  |  |  |  |  |  |
Comprehensive income (loss) | (146.0) |  | (141.2) | (4.5) | (145.7) | (50.5) | (196.2) |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (3.8) |  | (3.8) |  | (3.8) |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  |  |  |  |  | (8.0) | (8.0) |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 2.0 |  |  | 2.0 |  | 2.0 |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (519.1) | (105.8) | (413.3) |  | (519.1) | 0.0 | (519.1) |  |  |  |  |  |  |  |  |  |
Capital contributions from non-controlling interests |  |  |  |  |  |  |  |  |  |  |  | 45.0 | 45.0 |  |  |  |
Amortization of restricted share and option awards |  | 13.0 |  |  | 13.0 | 0.0 | 13.0 |  |  |  |  |  |  |  |  |  |
Acquisition of non-controlling interests |  | 7.4 |  |  | 7.4 | 4.3 | 11.7 |  |  |  |  |  |  |  |  |  |
Dilution from equity units of subsidiary |  | (1.5) |  |  | (1.5) | 1.5 | 0.0 |  |  |  |  |  |  |  |  |  |
Acquisition of non-controlling interests |  |  |  |  |  | 14.5 | 14.5 |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  | (1.7) |  |  | (1.7) |  | (1.7) |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2018 | 2,843.1 | 584.0 | 2,264.9 | (5.8) | 2,843.1 | (124.9) | 2,718.2 |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) | 376.6 |  | 414.5 |  | 414.5 | (37.9) | 376.6 |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and interest rate swap |  |  |  | (1.4) | (1.4) | 0.0 | (1.4) |  |  |  |  |  |  |  |  |  |
Comprehensive income (loss) | 413.1 |  | 414.5 | (1.4) | 413.1 | (37.9) | 375.2 |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (3.2) |  | (3.2) |  | (3.2) |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  |  |  |  |  | (2.0) | (2.0) |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 2.2 |  |  | 2.2 |  | 2.2 |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (4.9) | (1.1) | (3.8) |  | (4.9) |  | (4.9) |  |  |  |  |  |  |  |  |  |
Capital contributions from non-controlling interests |  | (0.9) |  |  | (0.9) | 2.4 | 1.5 |  |  |  |  | $ 57.6 | $ 57.6 |  |  |  |
Amortization of restricted share and option awards |  | 10.6 |  |  | 10.6 |  | 10.6 |  |  |  |  |  |  |  |  |  |
Acquisition of non-controlling interests |  | 1.5 |  |  | 1.5 | 0.0 | 1.5 |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests |  |  |  |  |  | (13.8) | (13.8) |  |  |  |  |  |  |  |  |  |
Acquisition of non-controlling interests |  | 0.0 |  |  | 0.0 | 1.8 | 1.8 |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2019 | $ 3,261.5 | $ 596.3 | $ 2,672.4 | $ (7.2) | $ 3,261.5 | $ (116.8) | $ 3,144.7 |  |  |  |  |  |  |  |  |  |

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- Definition Noncontrolling Interest Increase from Acquisition of Subsidiary + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Noncontrolling Interest, Increase from Contributions, Net + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Noncontrolling Interest, Recognition of Equity-Based Shares + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration