Accounts receivable - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Revenue recognized in excess of billings on services contracts or unbilled accounts receivable	$ 63.3	$ 60.8
Allowance for doubtful accounts	9.2	11.8
Provision for doubtful accounts reported in selling, general and administrative expenses, (income) expense	$ (0.3)	$ (1.6)	$ (5.1)