Significant Accounting Policies - Noncurrent Deferred Tax Assets Recorded in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Balance at beginning of period, Pension and Postretirement Benefit Plans	$ 87.9	$ 83.6	$ 89.4	$ 85.2
Tax effect of other comprehensive earnings, Pension and Postretirement Benefit Plans	(0.8)	(1.1)	(2.3)	(2.7)
Balance at end of period, Pension and Postretirement Benefit Plans	$ 87.1	$ 82.5	$ 87.1	$ 82.5