Revenue from contracts with customers - Contract balances (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables	$ 679.1	$ 635.6
Increase (Decrease) in Receivables	(45.1)	(15.8)	$ 49.7
Customer Asset	0.4
Contract liability	15.1	12.8
Revenue from performance obligation in prior periods	1.5
Pro Forma under Old Revenue Recognition Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables		635.6
Customer Asset		1.9
Contract liability		$ 12.8
Revenue recognized included in beginning balance	12.8
Accounting Standards Update 2014-09 \| Impact of New Revenue Recognition Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (Decrease) in Receivables	43.5
Contract Asset - increase (decrease)	(1.5)
Contract Liability - increase (decrease)	$ 2.3

Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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