Leases and commitments - Components of Lease Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 26.0	$ 36.7	$ 39.7
Finance lease cost
Amortization of right-of-use assets	0.2	1.2	1.8
Interest on lease liabilities	0.0	0.0	0.1
Total finance lease cost	0.2	1.2	1.9
Short-term lease costs	0.7	0.7	0.9
Variable lease cost	10.3	11.6	11.5
Sublease income	(1.1)	(1.8)	(4.4)
Total lease cost	$ 36.1	$ 48.4	$ 49.6