Income taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 2.1	$ 7.0	$ 8.7
Additions based on tax provisions related to the current year	0.2	0.1	0.1
Additions for tax provisions of prior years	0.5	0.0	0.0
Reductions resulting from a lapse in the statute of limitations	(0.3)	(0.3)	(0.3)
Reductions of tax provisions of prior years	(0.0)	(1.8)	(1.5)
Reductions resulting from audit closures	(0.0)	(2.9)	(0.0)
Balance at end of year	$ 2.5	$ 2.1	$ 7.0