Other Balance Sheet Accounts (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Current asset, related party	$ 173.9	$ 0.0
Nontrade Receivables, Current	24.1	37.9
Derivative Asset, Current	15.6	21.4
Income Taxes Receivable	4.2	10.1
Other Assets, Miscellaneous, Current	17.3	17.3
Other current assets	235.1	86.7
Other Current Liabilities
Current liability, related party	173.9	0.0
Accrued compensation	53.6	62.4
Deferred Revenue, Current	35.0	40.6
Environmental contingencies (see Note 10)	17.8	19.5
Pension liabilities included in other current liabilities	14.8	14.7
Accrued interest (see Note 5)	13.3	13.3
Operating Lease, Liability, Current	9.3	0.0
Income taxes payable (see Note 7)	8.6	11.3
Liability for dam spillway replacement, current	4.7	0.0
Other accrued liabilities	88.7	81.7
Other Liabilities, Current	419.7	243.5
Other Liabilities, Noncurrent [Abstract]
Environmental contingencies (see Note 10)	97.5	106.9
Liability for dam spillway replacement, noncurrent	61.7	0.0
Deferred revenue (see Note 17)	29.5	29.2
Operating Lease, Liability, Noncurrent	26.2	0.0
Legacy Product Liability	24.0	0.0
Derivative Liability, Noncurrent	13.2	32.6
Retained obligations of divested businesses	12.7	10.0
Asset Retirement Obligation	9.4	8.8
Deferred income taxes	7.5	10.9
Noncurrent liability	0.0	98.8
Other noncurrent liabilities	22.4	22.6
Other Liabilities, Noncurrent	308.2	319.8
Unrecognized Tax Benefits, Net of Deferred Taxes	$ 4.1	$ 0.0