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SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Revenues (loss) (including realized and unrealized gains and losses) | $ 2,166.7 | $ 1,157.9 | $ 614.4
Expenses | 1,601.3 | 1,307.3 | 888.0
Pre-tax income (loss) from continuing operations | 565.4 | (149.4) | (273.6)
Income tax (expense) benefit | 15.5 | (41.4) | (44.4)
Net income (loss) from continuing operations | 580.9 | (190.8) | (318.0)
Net income (loss) from discontinued operations, net of tax - NSM Group |  | 902.5 | (2.9)
Net income (loss) attributable to White Mountains’s common shareholders | 509.2 | 792.8 | (275.4)
Other comprehensive income (loss), net of tax | 2.4 | (3.8) | 1.7
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group | 0.0 | (5.2) | 0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group | 0.0 | (2.9) | 0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders | 511.1 | 787.6 | (273.3)
White Mountains Insurance Group Ltd. |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Revenues (loss) (including realized and unrealized gains and losses) | 81.7 | (0.4) | (0.2)
Expenses | 60.5 | 67.1 | 39.0
Pre-tax income (loss) from continuing operations | 21.2 | (67.5) | (39.2)
Income tax (expense) benefit | (1.2) | (0.9) | 0.0
Net income (loss) from continuing operations | 20.0 | (68.4) | (39.2)
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax | 489.2 | (42.0) | (232.3)
Net income (loss) from discontinued operations, net of tax - NSM Group | 0.0 | 16.4 | (22.6)
Net income (loss) attributable to White Mountains’s common shareholders | 509.2 | 792.8 | (275.4)
Other comprehensive income (loss), net of tax | 1.9 | (2.9) | 1.9
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group | 0.0 | (5.2) | 0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group | 0.0 | 2.9 | 0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders | 511.1 | 787.6 | (273.3)
Sirius Group |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group | 0.0 | 0.0 | 18.7
Loss contingency accrual, period increase (decrease) |  |  | 18.7
Sirius Group | White Mountains Insurance Group Ltd. |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group | 0.0 | 0.0 | 18.7
NSM |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group | 0.0 | 886.8 | 0.0
NSM | White Mountains Insurance Group Ltd. |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group | $ 0.0 | $ 886.8 | $ 0.0

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