Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Line Items]
Valuation allowance			$ 139.9				$ 109.6				$ 139.9	$ 109.6
Change in valuation allowance											$ 31.0	$ (42.6)	$ (6.9)
U.S. statutory rate (as a percent)	21.00%	35.00%									21.00%	35.00%
Income tax (expense) benefit			3.6	$ 3.6	$ (2.5)	$ (0.7)	2.5	$ 4.0	$ 1.0	$ 0.3	$ 4.0	$ 7.8	$ 32.9
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance			102.6				74.8				102.6	74.8
Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			21.1								21.1
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			14.5				13.5				14.5	13.5
Net Operating Losses in Netherlands Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			1.7								1.7
Luxembourg | U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Change in valuation allowance											1.2	6.4
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											2.1	3.0
United Kingdom | Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance											0.7	0.7
Subsidiaries | Luxembourg | Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance							20.3					20.3
Subsidiaries | United Kingdom | Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance							$ 1.0					1.0
Guilford Holdings, Inc. | United States
Valuation Allowance [Line Items]
Change in valuation allowance											22.7	(21.5)
U.S. statutory rate (as a percent)	21.00%	35.00%
Provisional income tax expense												20.4
BAM
Valuation Allowance [Line Items]
Change in valuation allowance											1.5	(18.4)
Provisional income tax expense												22.7
Income tax (expense) benefit											(8.7)	$ (10.1)
White Mountains Catskill Holdings, Inc.
Valuation Allowance [Line Items]
Valuation allowance			$ 2.8								$ 2.8