Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Trade Receivables, Net
Allowance for doubtful receivables	$ (0.7)	$ (0.8)
Trade receivables, net	389.2	112.8
Inventories
Finished products	59.5	37.2
Work-in-process	160.1	46.0
Raw materials	155.9	62.3
Operating supplies	7.7	6.5
Total	383.2	152.0
Property, Plant and Equipment, Net
Land and improvements	26.6	21.5
Buildings and leasehold improvements	176.3	117.2
Machinery and equipment	1,106.3	847.4
Construction in progress	74.6	37.8
Property, plant and equipment, gross	1,383.8	1,023.9
Accumulated depreciation and amortization	(433.9)	(398.4)
Assets held for sale	2.0	1.7
Property, plant and equipment, net	951.9	627.2
Other Accrued Liabilities
Uncleared cash disbursements	7.0	2.1
Accrued income taxes and other taxes payable	9.9	5.2
Accrued annual contribution to Salaried VEBA		1.7
Accrued interest	10.6	11.6
Short-term environmental accrual	3.6	3.7
Other	24.4	17.1
Total	55.5	41.4
Long-Term Liabilities
Workers' compensation accrual	31.3	30.6
Long-term environmental accrual	13.8	15.1
Other long-term liabilities	30.7	31.6
Total	75.8	77.3
Billed
Trade Receivables, Net
Billed trade receivables	$ 389.9	$ 113.6