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Investments in Unconsolidated Entities - Narrative (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019 | Mar. 31, 2020 | Feb. 26, 2019 | Dec. 31, 2018
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 53.80% |  |  | 53.80% |  | 50.00% | 50.00% |  |
Total revenues |  | $ 259,600,000 | $ 129,000,000.0 |  | $ 200,400,000 | $ 562,600,000 |  |  |  |
Benefits, Losses and Expenses |  | 126,900,000 | 113,200,000 |  | 233,700,000 | 265,100,000 |  |  |  |
Total assets |  | 4,000,300,000 |  |  | 4,000,300,000 |  | $ 3,983,200,000 |  |  |
Total liabilities |  | 937,000,000.0 |  |  | 937,000,000.0 |  | 838,500,000 |  |  |
Net income (loss) attributable to White Mountains’s common shareholders |  | 115,400,000 | 20,500,000 |  | (13,400,000) | 304,900,000 |  |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  | 48.30% | 61.00%
Fully diluted ownership interest (as a percent) |  |  |  |  |  |  |  |  | 42.00% | 58.90%
Assets, fair value | $ 114,700,000 |  |  | $ 114,700,000 |  |  |  |  | $ 114,700,000 |
Unrealized investment gain |  | 15,000,000.0 |  | $ 114,700,000 | 45,000,000.0 | 114,700,000 | 180,000,000.0 |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Total revenues |  | 0 | 0 |  | 0 | 48,800,000 |  |  |  |
Benefits, Losses and Expenses |  | 0 | 0 |  | 0 | 54,900,000 |  |  |  |
MediaAlpha, Kudu and Tuckerman |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Total revenues |  | 118,700,000 | 93,700,000 |  | 307,200,000 | 178,400,000 |  |  |  |
Expenses |  | 131,900,000 | 83,600,000 |  | 273,800,000 | 171,100,000 |  |  |  |
Total assets |  | 420,500,000 |  |  | 420,500,000 |  | 457,000,000.0 |  |  |
Total liabilities |  | 200,200,000 |  |  | 200,200,000 |  | $ 199,000,000.0 |  |  |
Net income (loss) attributable to White Mountains’s common shareholders |  | $ (21,800,000) | $ 10,100,000 |  | $ 21,800,000 | $ 7,300,000 |  |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 48.30% |  |  | 48.30% |  | 48.30% |  |  |
Total revenues | 122,700,000 |  |  |  |  |  |  |  |  |
Benefits, Losses and Expenses | 115,800,000 |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders | $ 6,900,000 |  |  |  |  |  |  |  |  |

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