Leases and commitments - Components of Lease Expense (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 69.8
Finance lease cost
Amortization of right-of-use assets	1.6
Interest on lease liabilities	0.3
Total finance lease cost	1.9
Short-term lease costs	0.6
Variable lease cost	16.6
Sublease income	(0.7)
Total lease cost	$ 88.2