Defined Benefit Plans - Obligations and Funded Status (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Change in the Projected Benefit Obligation (PBO):
Recognition of actuarial losses (gains)		$ 0.8	$ 3.2	$ 34.1
Change in Plan Assets:
Beginning balance		784.9
Ending balance		816.0	784.9
Pension Benefits [Member]
Change in the Projected Benefit Obligation (PBO):
Projected benefit obligation at beginning of year		933.5	877.1
Service cost		0.4	0.4
Interest cost		24.0	28.3
Recognition of actuarial losses (gains)		(32.4)	70.6
Benefits paid		(40.2)	(42.9)
Projected benefit obligation at end of year		885.3	933.5	877.1
Accumulated benefit obligation at end of year (excludes the impact of future compensation increases)		885.3	933.5
Change in Plan Assets:
Beginning balance		784.9	677.2
Actual return on plan assets		48.4	101.3
Employer contributions		22.9	49.3
Benefits paid		(40.2)	(42.9)
Ending balance		816.0	784.9	677.2
Funded status (Fair value of plan assets less PBO)		(69.3)	(148.6)
Postretirement Benefits [Member]
Change in the Projected Benefit Obligation (PBO):
Projected benefit obligation at beginning of year		13.4	3.6
Projected benefit obligation acquired	[1]		9.6
Service cost		0.6	0.4
Interest cost		0.4	0.2
Recognition of actuarial losses (gains)		(0.7)	0.2
Benefits paid		(0.4)	(0.6)
Projected benefit obligation at end of year		13.3	13.4	$ 3.6
Change in Plan Assets:
Employer contributions		0.4	0.6
Benefits paid		(0.4)	(0.6)
Funded status (Fair value of plan assets less PBO)		$ (13.3)	$ (13.4)

[1] Related to the Larson acquisition discussed in Note 4.