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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, 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
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | $ 109.6 |  |  |  | $ 146.2 |  |  |  | $ 109.6 | $ 146.2 |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | $ (42.6) | (6.9) | $ 21.8
U.S. statutory rate (as a percent) | 21.00% | 35.00% |  |  |  |  |  |  |  |  | 35.00% |  |
Income Tax Expense (Benefit) |  |  | (2.5) | $ (4.0) | $ (1.0) | $ (0.3) | (10.2) | $ (17.1) | $ (4.0) | $ (1.6) | $ (7.8) | (32.9) | $ 12.7
U.S. losses and other federal deferred tax benefits |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 74.8 |  |  |  | 112.5 |  |  |  | 74.8 | 112.5 |
United Kingdom | Net operating losses in UK Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 0.7 | 0.1 |
Unites States |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense |  |  |  |  |  |  |  |  |  |  | 43.1 |  |
Unites States | U.S. losses and other federal deferred tax benefits |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 6.4 |  |
Non-US | Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  |  | 2.8 |
Israel | Net operating losses in Israel Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | 3.0 | 3.4 |
Subsidiaries | Net operating losses in Israel Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 13.5 |  |  |  | 9.4 |  |  |  | 13.5 | 9.4 |
Subsidiaries | Luxembourg | Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | 20.3 |  |  |  | 24.0 |  |  |  | 20.3 | 24.0 |
Subsidiaries | United Kingdom | Net operating losses in UK Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  | $ 1.0 |  |  |  | $ 0.3 |  |  |  | 1.0 | 0.3 |
Guilford Holdings, Inc. | Unites States |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | (21.5) | 17.1 |
U.S. statutory rate (as a percent) | 21.00% | 35.00% |  |  |  |  |  |  |  |  |  |  |
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense |  |  |  |  |  |  |  |  |  |  | 20.4 |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | (18.4) | 3.6 |
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense |  |  |  |  |  |  |  |  |  |  | 22.7 |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | (10.1) | (11.0) |
Star & Shield Insurance Exchange |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in valuation allowance |  |  |  |  |  |  |  |  |  |  |  | $ (6.9) |
Continuing Operations | Guilford Holdings, Inc. | Unites States |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  |  |  | $ (21.4) |  |

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