Property, plant and equipment, net (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Property, plant and equipment, net
Property, plant and equipment, gross	$ 924.7	$ 893.7
Less accumulated depreciation	(432.9)	(391.6)
Total property, plant and equipment, net	491.8	502.1
Depreciation expense	36.8	37.6
Impairment of property, plant and equipment	0.0	4.0
Land
Property, plant and equipment, net
Property, plant and equipment, gross	6.4	6.4
Office equipment, machinery and other
Property, plant and equipment, net
Property, plant and equipment, gross	$ 6.7	6.5
Office equipment, machinery and other \| Minimum
Property, plant and equipment, net
Depreciable Lives	3 years
Office equipment, machinery and other \| Maximum
Property, plant and equipment, net
Depreciable Lives	10 years
Leasehold improvements
Property, plant and equipment, net
Property, plant and equipment, gross	$ 2.1	2.1
Leasehold improvements \| Minimum
Property, plant and equipment, net
Depreciable Lives	7 years
Leasehold improvements \| Maximum
Property, plant and equipment, net
Depreciable Lives	15 years
Asset retirement obligation.
Property, plant and equipment, net
Property, plant and equipment, gross	$ 23.6	23.4
Asset retirement obligation. \| Minimum
Property, plant and equipment, net
Depreciable Lives	1 year
Asset retirement obligation. \| Maximum
Property, plant and equipment, net
Depreciable Lives	43 years
Plant in service
Property, plant and equipment, net
Property, plant and equipment, gross	$ 885.1	848.1
Plant in service \| Minimum
Property, plant and equipment, net
Depreciable Lives	1 year
Plant in service \| Maximum
Property, plant and equipment, net
Depreciable Lives	45 years
Construction in progress
Property, plant and equipment, net
Property, plant and equipment, gross	$ 0.8	$ 7.2

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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