Accounts receivable - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Revenue recognized in excess of billings on services contracts or unbilled accounts receivable	$ 102.8	$ 94.4
Unearned income deducted from accounts receivable	8.7	8.4
Allowance for doubtful accounts	11.8	13.7
Provision for doubtful accounts reported in selling, general and administrative expenses, (income) expense	$ (1.6)	$ (5.1)	$ 3.1