Other Balance Sheet Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	$ 158.1	$ 0.0
Nontrade Receivables, Current	30.7	37.9
Derivative Asset, Current	16.5	21.4
Income Taxes Receivable	8.0	10.1
Other Assets, Miscellaneous, Current	17.2	17.3
Other current assets	230.5	86.7
Other Current Liabilities
Current liability, related party	158.1	0.0
Employee-related Liabilities, Current	48.7	62.4
Deferred revenue (see Note 13)	36.1	40.6
Accrued interest (see Note 3)	26.4	13.3
Environmental contingencies (see Note 8)	18.7	19.5
Pension liabilities (see Note 6)	14.3	14.7
Income taxes payable (see Note 5)	13.9	11.3
Operating Lease, Liability, Current	9.5	0.0
Liability for dam spillway replacement, current	7.7	0.0
Other accrued liabilities	82.0	81.7
Total Other Current Liabilities	415.4	243.5
Other Liabilities, Noncurrent
Environmental contingencies (see Note 8)	99.5	106.9
Liability for dam spillway replacement	36.8	0.0
Deferred revenue (see Note 13)	28.8	29.2
Operating Lease, Liability, Noncurrent	22.7	0.0
Retained obligations of divested businesses	13.0	10.0
Asset retirement obligation	9.3	8.8
Deferred income taxes	6.8	10.9
Postemployment liability	4.4	4.7
Derivative Liability, Noncurrent	3.5	32.6
Liability to unconsolidated affiliate (see Note 15)	0.0	98.8
Other noncurrent liabilities	14.9	17.9
Other Liabilities, Noncurrent	$ 239.7	$ 319.8