Leases - Aggregate lease maturities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Finance Leases
First year	$ 15.5	$ 8.9
Second year	11.8	8.8
Third year	5.8	5.1
Fourth year	1.3	1.2
Fifth year	0.4	0.5
Total lease payments, Finance leases	34.8	24.5
Less: Interest	1.9	1.4
Present value of lease liabilities	32.9	23.1
Operating Leases
First year	8.0	7.7
Second year	7.1	6.9
Third year	5.3	6.1
Fourth year	4.3	4.5
Fifth year	2.9	3.1
Thereafter	4.7	6.8
Total lease payments, Operating leases	32.3	35.1
Less: Interest	4.5	5.6
Present value of lease liabilities	$ 27.8	$ 29.5
Operating Lease, Liability, Statement of Financial Position	us-gaap:OperatingLeaseLiabilityCurrent us-gaap:OperatingLeaseLiabilityNoncurrent	us-gaap:OperatingLeaseLiabilityCurrent us-gaap:OperatingLeaseLiabilityNoncurrent
Total Finance and Operating Leases
First year	$ 23.5	$ 16.6
Second year	18.9	15.7
Third year	11.1	11.2
Fourth year	5.6	5.7
Fifth year	3.3	3.6
Thereafter	4.7	6.8
Total lease payments	67.1	59.6
Less: Interest	6.4	7.0
Present value of lease liabilities	$ 60.7	$ 52.6