Discontinued operations - Summary (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income (loss)
Income from discontinued operations, net of tax	$ 0.0	$ 1,068.4	$ 75.0
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]
Revenue		149.5	725.9
Income (loss)
Operations		8.4	100.3
Gain on sale		1,060.7	0.0
Income (loss) from discontinued operation		1,069.1	100.3
Income tax provision		0.7	25.3
Income from discontinued operations, net of tax		$ 1,068.4	$ 75.0

Amount before tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation.

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