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Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 22, 2017 | Dec. 21, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. statutory rate (as a percent) | 21.00% | 35.00% |  |  |  |  |  |  |  |  | 21.00% | 35.00% |
Valuation allowance |  |  | $ 77.9 |  |  |  | $ 139.9 |  |  |  | $ 77.9 | $ 139.9 |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | (63.6) | 31.0 | $ (42.6)
Income tax (expense) benefit |  |  | (10.4) | $ (8.8) | $ 0.1 | $ (10.2) | 3.6 | $ 3.6 | $ (2.5) | $ (0.7) | (29.3) | 4.0 | $ 7.8
U.S. losses and other federal deferred tax benefits |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 34.9 |  |  |  | 102.6 |  |  |  | 34.9 | 102.6 |
Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 22.6 |  |  |  |  |  |  |  | 22.6 |  |
Net Operating Losses in Israel Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 17.3 |  |  |  | 14.5 |  |  |  | 17.3 | 14.5 |
Net Operating Losses in Netherlands Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 3.1 |  |  |  |  |  |  |  | 3.1 |  |
Luxembourg | U.S. losses and other federal deferred tax benefits |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 1.1 | 1.2 |
United States |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Provisional income tax expense |  |  |  |  |  |  |  |  |  |  | 43.1 |  |
Israel | Net Operating Losses in Israel Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 1.6 | 2.1 |
United Kingdom | Net Operating Losses in United Kingdom Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 1.3 | 0.7 |
Subsidiaries | Luxembourg | Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  | 21.1 |  |  |  |  | 21.1 |
Subsidiaries | United Kingdom | Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  | 1.7 |  |  |  |  | 1.7 |
Guilford Holdings, Inc. | United States |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | (63.2) | 22.7 |
BAM |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | (4.5) | 1.5 |
Income tax (expense) benefit |  |  |  |  |  |  |  |  |  |  | 10.5 | 8.7 |
White Mountains Catskill Holdings, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | $ 0.1 |  |  |  | $ 2.8 |  |  |  | $ 0.1 | $ 2.8 |

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