Leases and commitments - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Cash payments for operating leases included in operating activities	$ 41.6	$ 42.1
Cash payments for finance leases included in financing activities	1.8	1.7
Cash payments for finance lease included in operating activities	0.2	0.3
Right-of-use assets obtained in exchange for lease obligations:
Operating leases	40.9	17.4
Finance leases	$ 0.0	$ 1.5