Retirement Plans - Components of Net Periodic Pension Benefit Cost and Other Changes in Plan Assets and Obligations Recognized in Earnings and Other Comprehensive Earnings (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net periodic pension cost:
Service cost	$ 0.8	$ 1.6	$ 0.8
Interest cost on benefit obligation	8.0	9.8	9.3
Expected return on plan assets	(16.4)	(14.8)	(16.0)
Amortization of net loss	6.2	7.0	4.9
Net periodic benefit cost	(1.4)	3.6	(1.0)
Other changes in plan assets and obligations recognized in other comprehensive earnings:
Net loss incurred	4.7	0.8	17.2
Amortization of net loss	(6.2)	(7.0)	(4.9)
Total recognized in other comprehensive loss	(1.5)	(6.2)	12.3
Total recognized in net periodic pension cost and other comprehensive loss	(2.9)	(2.6)	11.3
Estimated amortization for the following year:
Amortization of net loss	$ 5.8	$ 6.1	$ 7.2