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Income Taxes - Valuation Allowance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation Allowance [Line Items]
Valuation allowance	$ 97.4				$ 77.9				$ 97.4	$ 77.9
Change in valuation allowance									29.2	(63.6)	$ 31.0
Income tax benefit (expense)	117.6	$ (98.5)	$ (24.1)	$ 25.5	(10.4)	$ (8.8)	$ 0.1	$ (10.2)	20.5	(29.3)	$ 4.0
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance	46.5				34.9				46.5	34.9
Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	26.8								26.8
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	20.0				17.3				20.0	17.3
Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	$ 4.1								4.1
Luxembourg \| U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Change in valuation allowance									4.2	1.1
Israel \| Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance									2.7	1.6
United Kingdom \| Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance									1.0	1.3
Subsidiaries \| Luxembourg \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					22.6					22.6
Subsidiaries \| United Kingdom \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					$ 3.1					3.1
Guilford Holdings, Inc. \| United States
Valuation Allowance [Line Items]
Change in valuation allowance									14.9
Income tax benefit (expense)										63.2
BAM \| United States
Valuation Allowance [Line Items]
Income tax benefit (expense)									(4.5)	4.5
BAM \| United States \| MSC [Member]
Valuation Allowance [Line Items]
Income tax benefit (expense)									9.7	10.5
BAM \| United States \| Foreign Tax Credits
Valuation Allowance [Line Items]
Income tax benefit (expense)									$ (1.9)	$ (0.1)