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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

X

- Definition

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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