Leases and commitments - Components of Lease Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 42.3	$ 37.9
Finance lease cost
Amortization of right-of-use assets	1.7	1.6
Interest on lease liabilities	0.2	0.3
Total finance lease cost	1.9	1.9
Short-term lease costs	1.4	0.6
Variable lease cost	10.3	13.7
Sublease income	(12.1)	(0.7)
Total lease cost	$ 43.8	$ 53.4