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Quarterly financial information (unaudited) - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
	Mar. 13, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of U.S. Federal business										$ 1,162.9	$ 0.0	$ 0.0
Non-cash pension settlement charge		$ 142.1
Debt extinguishment charge				$ 28.5			$ 20.1			28.5	20.1	0.0
Restructuring charges (benefits)		$ 46.9	$ 13.2	$ 7.9	$ 27.5	$ 23.3	$ 0.2	$ 2.6	$ 2.6	$ 95.5	$ 28.7	$ 19.7
U.S. Federal Business \| Discontinued Operation, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of U.S. Federal business	$ 1,200.0