Other Balance Sheet Accounts (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	$ 184.7	$ 173.9
Nontrade Receivables, Current	21.6	24.1
Derivative Asset, Current	10.9	15.6
Income Taxes Receivable	9.5	4.2
Other Assets, Miscellaneous, Current	18.4	17.3
Other current assets	245.1	235.1
Other Current Liabilities
Current liability, related party	184.7	173.9
Employee-related Liabilities, Current	35.3	53.6
Deferred revenue (see Note 13)	34.8	35.0
Accrued interest (see Note 3)	26.4	13.3
Environmental contingencies (see Note 8)	18.7	17.8
Pension liabilities (see Note 6)	14.7	14.8
Income taxes payable (see Note 5)	11.5	8.6
Operating Lease, Liability, Current	8.9	9.3
Liability for dam spillway replacement, current	4.4	4.7
Other accrued liabilities	75.2	88.7
Total Other Current Liabilities	414.6	419.7
Other Liabilities, Noncurrent
Environmental contingencies (see Note 8)	94.1	97.5
Liability for dam spillway replacement	61.0	61.7
Deferred revenue (see Note 13)	30.0	29.5
Operating Lease, Liability, Noncurrent	25.4	26.2
Legacy Product Liability	24.0	24.0
Retained obligations of divested businesses	11.9	12.7
Asset retirement obligation	9.5	9.4
Deferred Income Tax Liabilities, Net	7.5	7.5
Derivative Liability, Noncurrent	4.2	13.2
Unrecognized Tax Benefits, Net of Deferred Taxes	4.0	4.1
Other noncurrent liabilities	22.2	22.4
Other Liabilities, Noncurrent	$ 293.8	$ 308.2