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Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  | $ 87.5 | $ 83.5 | $ 88.8 | $ (5.5) | $ 56.8 | $ 48.4 | $ 58.0 | $ 373.8 | $ 157.7 | $ 440.0
Advertising & commission revenues |  |  |  |  |  |  |  |  | 167.0 | 118.3 | 107.4
Total expenses |  | 79.1 | 85.7 | 92.4 | 69.0 | 70.6 | 74.1 | 91.3 | 366.0 | 305.0 | 310.8
Amortization of other intangible assets |  |  |  |  |  |  |  |  | 10.7 | 10.5 | 8.4
Pre-tax (loss) income from continuing operations | $ 5.2 | 8.4 | (2.2) | (3.6) | (74.5) | (13.8) | (25.7) | (33.3) | 7.8 | (147.3) | 129.2
Income tax benefit (expense) | 2.5 | 4.0 | 1.0 | 0.3 | 10.2 | 17.1 | 4.0 | 1.6 | 7.8 | 32.9 | (12.7)
Net income (loss) from continuing operations | 7.7 | 12.4 | (1.2) | (3.3) | (64.3) | 3.3 | (21.7) | (31.7) | 15.6 | (114.4) | 116.5
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  | 557.0 | 415.1 | 18.2
Net (loss) income from discontinued operations, net of tax | 4.3 | 539.1 | 2.8 | 31.3 | 8.0 | 84.4 | 383.6 | 47.4 | 20.5 | 108.3 | 116.9
Income before equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  | 593.1 | 409.0 | 251.6
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 25.1
Net income |  |  |  |  |  |  |  |  | 593.1 | 409.0 | 276.7
Net (income) loss attributable to non-controlling interests | 10.5 | 10.6 | 12.0 | 1.0 | 17.4 | 3.1 | (21.5) | (6.2) | 34.1 | (7.2) | 18.5
Net income attributable to White Mountains’s common shareholders (3) | $ 22.5 | $ 562.1 | 13.6 | 29.0 | (38.9) | $ 90.8 | 340.4 | 9.5 | 627.2 | 401.8 | 295.2
Other comprehensive income (loss), net of tax |  |  |  |  |  |  |  |  | 0.3 | (0.7) | (42.8)
Comprehensive income |  |  |  |  |  |  |  |  | 630.7 | 547.4 | 187.4
Comprehensive (income) loss attributable to non-controlling interests |  |  |  |  |  |  |  |  | (0.2) | (0.3) | 0.0
Comprehensive income attributable to White Mountains’s common shareholders |  |  |  |  |  |  |  |  | 630.5 | $ 547.1 | $ 187.4
Basic and diluted earnings per share - continuing operations |  |  |  |  |  |  |  |  |  | $ (24.26) | $ 27.22
Previously Reported |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  | 85.4 | 93.5 | 0.3 |  | 49.2 | 58.8 |  | $ 166.3 | $ 435.3
Total expenses |  |  | 85.8 | 92.2 | 69.1 |  | 74.3 | 88.5 |  | 302.8 | 303.3
Pre-tax (loss) income from continuing operations |  |  | (0.4) | 1.3 | (68.8) |  | (25.1) | (29.7) |  | (136.5) | 132.0
Income tax benefit (expense) |  |  | 1.0 | 0.3 | 10.2 |  | 4.0 | 1.6 |  | 32.9 | (12.7)
Net income (loss) from continuing operations |  |  | 0.6 | 1.6 | (58.6) |  | (21.1) | (28.1) |  | (103.6) | 119.3
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  | 415.1 | 18.2
Net (loss) income from discontinued operations, net of tax |  |  | 2.8 | 31.3 | 8.0 |  | 383.6 | 47.4 |  | 108.3 | 116.9
Income before equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  |  | 419.8 | 254.4
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  |  | 0.0 | 25.1
Net income |  |  |  |  |  |  |  |  |  | 419.8 | 279.5
Net (income) loss attributable to non-controlling interests |  |  | 12.1 | 1.3 | 17.3 |  | (21.4) | (6.3) |  | (7.3) | 18.1
Net income attributable to White Mountains’s common shareholders (3) |  |  | 15.5 | 34.2 | (33.3) |  | 341.1 | 13.0 |  | 412.5 | 297.6
Comprehensive income |  |  |  |  |  |  |  |  |  | 558.1 | 197.2
Comprehensive (income) loss attributable to non-controlling interests |  |  |  |  |  |  |  |  |  | (0.3) | 0.0
Comprehensive income attributable to White Mountains’s common shareholders |  |  |  |  |  |  |  |  |  | $ 557.8 | $ 197.2
Basic and diluted earnings per share - continuing operations |  |  |  |  |  |  |  |  |  | $ (22.13) | $ 27.63
Adjustment |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  | (1.9) | (4.7) | (5.8) |  | (0.8) | (0.8) |  | $ (8.6) | $ 4.7
Total expenses |  |  | (0.1) | 0.2 | (0.1) |  | (0.2) | 2.8 |  | 2.2 | 7.5
Pre-tax (loss) income from continuing operations |  |  | (1.8) | (4.9) | (5.7) |  | (0.6) | (3.6) |  | (10.8) | (2.8)
Income tax benefit (expense) |  |  |  | 0.0 | 0.0 |  | 0.0 | 0.0 |  | 0.0 | 0.0
Net income (loss) from continuing operations |  |  | (1.8) | (4.9) | (5.7) |  | (0.6) | (3.6) |  | (10.8) | (2.8)
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Net (loss) income from discontinued operations, net of tax |  |  | 0.0 | 0.0 | 0.0 |  | 0.0 | 0.0 |  | 0.0 | 0.0
Income before equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  |  | (10.8) | (2.8)
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Net income |  |  |  |  |  |  |  |  |  | (10.8) | (2.8)
Net (income) loss attributable to non-controlling interests |  |  | (0.1) | (0.3) | 0.1 |  | (0.1) | 0.1 |  | 0.1 | 0.4
Net income attributable to White Mountains’s common shareholders (3) |  |  | $ (1.9) | $ (5.2) | $ (5.6) |  | $ (0.7) | $ (3.5) |  | (10.7) | (2.4)
Comprehensive income |  |  |  |  |  |  |  |  |  | (10.7) | (9.8)
Comprehensive (income) loss attributable to non-controlling interests |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Comprehensive income attributable to White Mountains’s common shareholders |  |  |  |  |  |  |  |  |  | $ (10.7) | $ (9.8)
Basic and diluted earnings per share - continuing operations |  |  |  |  |  |  |  |  |  | $ (2.13) | $ (0.41)
HG Global BAM |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 23.3 | $ 16.7 | $ 10.7
Total expenses |  |  |  |  |  |  |  |  | 47.3 | 43.4 | 40.1
HG Global BAM | Previously Reported |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | 16.7 | 10.7
Total expenses |  |  |  |  |  |  |  |  |  | 43.4 | 40.1
HG Global BAM | Adjustment |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Total expenses |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
MediaAlpha |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 163.2 | 116.5 | 105.5
Advertising & commission revenues |  |  |  |  |  |  |  |  | 163.2 | 116.5 | 105.5
Total expenses |  |  |  |  |  |  |  |  | 163.6 | 120.6 | 107.5
Amortization of other intangible assets |  |  |  |  |  |  |  |  | 10.5 | 10.1 | 8.1
MediaAlpha | Previously Reported |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | 116.5 | 105.5
Total expenses |  |  |  |  |  |  |  |  |  | 120.6 | 107.5
MediaAlpha | Adjustment |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Total expenses |  |  |  |  |  |  |  |  |  | 0.0 | 0.0
Other |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 187.3 | 24.5 | 323.8
Advertising & commission revenues |  |  |  |  |  |  |  |  | 3.8 | 1.8 | 1.9
Other revenues |  |  |  |  |  |  |  |  |  | 22.7 | 321.9
Total expenses |  |  |  |  |  |  |  |  | 155.1 | 141.0 | 163.2
Other expense |  |  |  |  |  |  |  |  |  | 140.6 | 162.7
Amortization of other intangible assets |  |  |  |  |  |  |  |  | 0.2 | 0.4 | 0.5
Other | Previously Reported |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | 33.1 | 319.1
Advertising & commission revenues |  |  |  |  |  |  |  |  |  | 10.4 | 4.6
Other revenues |  |  |  |  |  |  |  |  |  | 22.7 | 314.5
Total expenses |  |  |  |  |  |  |  |  |  | 138.8 | 155.7
Other expense |  |  |  |  |  |  |  |  |  | 137.6 | 154.5
Amortization of other intangible assets |  |  |  |  |  |  |  |  |  | 1.2 | 1.2
Other | Adjustment |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  | (8.6) | 4.7
Advertising & commission revenues |  |  |  |  |  |  |  |  |  | (8.6) | (2.7)
Other revenues |  |  |  |  |  |  |  |  |  | 0.0 | 7.4
Total expenses |  |  |  |  |  |  |  |  |  | 2.2 | 7.5
Other expense |  |  |  |  |  |  |  |  |  | 3.0 | 8.2
Amortization of other intangible assets |  |  |  |  |  |  |  |  |  | (0.8) | (0.7)
Discontinued Operations |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders (3) |  |  |  |  |  |  |  |  | $ 627.2 | 401.8 | 295.2
Other comprehensive income (loss), net of tax |  |  |  |  |  |  |  |  |  | 145.6 | (107.8)
Discontinued Operations | Previously Reported |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), net of tax |  |  |  |  |  |  |  |  |  | 145.6 | (100.4)
Discontinued Operations | Adjustment |  |  |  |  |  |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), net of tax |  |  |  |  |  |  |  |  |  | $ 0.0 | $ (7.4)

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