Stockholders' Equity - 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
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period			$ 682.5
Balance at end of period	$ 756.5		756.5
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	1.7	$ 1.8	1.8	$ 1.7
Other comprehensive income (loss)			(0.1)	0.1
Balance at end of period	1.7	1.8	1.7	1.8
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(64.8)	(92.4)	(67.4)	(76.8)
Other comprehensive income (loss)	(5.3)	6.8	(2.7)	(8.8)
Balance at end of period	(70.1)	(85.6)	(70.1)	(85.6)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(149.1)	(142.1)	(154.1)	(146.6)
Other comprehensive income (loss)	2.5	2.3	7.5	6.8
Balance at end of period	(146.6)	(139.8)	(146.6)	(139.8)
Accumulated Defined Benefit Plans Adjustment | OPEB
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	0.1	0.7	0.3	1.0
Other comprehensive income (loss)	(0.1)		(0.3)	(0.3)
Balance at end of period		0.7		0.7
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(212.1)	(232.0)	(219.4)	(220.7)
Other comprehensive income (loss)	(2.9)	9.1	4.4	(2.2)
Balance at end of period	$ (215.0)	$ (222.9)	$ (215.0)	$ (222.9)