Other Balance Sheet Accounts (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	$ 143.8	$ 0.0
Nontrade Receivables, Current	33.2	37.9
Derivative Asset, Current	17.0	21.4
Income Taxes Receivable	7.2	10.1
Other Assets, Miscellaneous, Current	18.4	17.3
Other current assets	219.6	86.7
Other Current Liabilities
Current liability, related party	143.8	0.0
Employee-related Liabilities, Current	42.1	62.4
Deferred revenue (see Note 13)	34.4	40.6
Environmental contingencies (see Note 8)	21.1	19.5
Income taxes payable (see Note 5)	18.1	11.3
Pension liabilities (see Note 6)	14.6	14.7
Accrued interest (see Note 3)	13.2	13.3
Operating Lease, Liability, Current	8.6	0.0
Liability for dam spillway replacement, current	6.0	0.0
Other accrued liabilities	84.3	81.7
Total Other Current Liabilities	386.2	243.5
Other Liabilities, Noncurrent
Environmental contingencies (see Note 8)	99.0	106.9
Liability for dam spillway replacement	38.8	0.0
Derivative Liability, Noncurrent	33.6	32.6
Deferred revenue (see Note 13)	24.3	29.2
Operating Lease, Liability, Noncurrent	20.1	0.0
Retained obligations of divested businesses	13.3	10.0
Deferred income taxes	7.1	10.9
Asset retirement obligation	8.1	8.8
Postemployment liability	4.5	4.7
Liability to unconsolidated affiliate (see Note 15)	0.0	98.8
Other noncurrent liabilities	17.0	17.9
Other Liabilities, Noncurrent	265.8	319.8
ART [Member]
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	143.8	0.0
Other Current Liabilities
Current liability, related party	143.8	0.0
Other Liabilities, Noncurrent
Liability to unconsolidated affiliate (see Note 15)	$ 0.0	$ 98.8