Stockholders' equity - Schedule of Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 816.1	$ 839.8	$ 754.3
Ending Balance	796.5	816.1	839.8
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(246.8)	(245.0)	(211.9)
Other comprehensive income (loss)	13.4	(1.8)	(33.1)
Ending Balance	(233.4)	(246.8)	(245.0)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(202.2)	(180.0)	(172.8)
Other comprehensive income (loss)	13.4	9.5	10.4
Net actuarial gain (loss) arising during year	(25.7)	(31.7)	(17.6)
Ending Balance	(214.5)	(202.2)	(180.0)
Accumulated Defined Benefit Plans Adjustment | OPEB Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	0.2	0.7	1.2
Other comprehensive income (loss)	(0.2)	(0.3)	(0.3)
Net actuarial gain (loss) arising during year	(0.3)	(0.2)	(0.2)
Ending Balance	(0.3)	0.2	0.7
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			4.8
Marketable Securities | Change in Accounting Principle - ASU 2016-01
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			(4.8)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(448.8)	(424.3)	(378.7)
Balance at beginning of period, as adjusted	(448.8)	(424.3)	(383.5)
Other comprehensive income (loss)	0.6	(24.5)	(40.8)
Ending Balance	$ (448.2)	$ (448.8)	(424.3)
Total Accumulated Other Comprehensive Loss | Change in Accounting Principle - ASU 2016-01
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			$ (4.8)