Retirement Plans and Other Post-Retirement Benefits - Schedule of Change in Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Change in Benefit Obligation
Balance at beginning of year	$ 45.7	$ 332.2
Service cost		1.3	$ 2.3
Interest cost	1.2	11.5	13.3
Benefits paid	(2.4)	(22.3)
Plan amendments		(0.2)
Actuarial (gain)/loss	1.7	29.2
Special termination benefits		1.3
Curtailment		(1.9)
Settlement/transfer		(309.5)
Transfers from Discontinued Operations		4.3
Effect of currency rate changes	1.1	(0.2)
Balance at end of year	47.3	45.7	332.2
Change in Plan Assets
Fair value of plan assets at beginning of year	53.4	333.2
Reversion of excess plan assets	(53.4)
Actual return on plan assets		44.1
Total contributions	2.4	2.2
Benefits paid	(2.4)	(22.3)
Transfers from Discontinued Operations		5.7
Settlement/transfer		(309.5)
Fair value of plan assets at end of year		53.4	333.2
Funded status at end of year	(47.3)	7.7
Other Benefits [Member]
Change in Benefit Obligation
Balance at beginning of year	6.5	9.2
Service cost			0.1
Interest cost	0.2	0.3	0.5
Benefits paid	(1.6)	(3.0)
Participant contributions		0.9
Actuarial (gain)/loss	0.9	(0.3)
Curtailment		(0.6)
Balance at end of year	6.0	6.5	$ 9.2
Change in Plan Assets
Total contributions	1.6	3.0
Benefits paid	(1.6)	(3.0)
Funded status at end of year	$ (6.0)	$ (6.5)