Other Balance Sheet Accounts (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	$ 114.5	$ 0.0
Nontrade Receivables, Current	31.4	37.9
Total Assets	6.3	3.7
Income Taxes Receivable	19.4	10.1
Other Assets, Miscellaneous, Current	16.3	17.3
Other current assets	202.8	86.7
Other Current Liabilities
Current liability, related party	114.5	0.0
Deferred revenue (see Note 13)	41.4	40.6
Employee-related Liabilities, Current	34.9	62.4
Accrued interest (see Note 3)	26.4	13.3
Environmental contingencies (see Note 8)	20.8	19.5
Income taxes payable (see Note 5)	18.0	11.3
Pension liabilities (see Note 6)	14.6	14.7
Operating Lease, Liability, Current	8.7	0.0
Liability for dam spillway replacement, current	4.2	0.0
Other accrued liabilities	72.5	81.7
Total Other Current Liabilities	356.0	243.5
Other Liabilities, Noncurrent
Environmental contingencies (see Note 8)	102.6	106.9
Liability for dam spillway replacement	40.8	0.0
Deferred revenue (see Note 13)	24.4	29.2
Derivative Liability, Noncurrent	22.8	32.6
Operating Lease, Liability, Noncurrent	21.3	0.0
Deferred income taxes	10.8	10.9
Asset retirement obligation	8.9	8.8
Postemployment liability	4.6	4.7
Liability to unconsolidated affiliate (see Note 15)	0.0	98.8
Other noncurrent liabilities	28.3	27.9
Other Liabilities, Noncurrent	264.5	319.8
Other current assets
Schedule of other assets and other liabilities [Line Items]
Total Assets	21.2	21.4
ART [Member]
Schedule of other assets and other liabilities [Line Items]
Current asset, related party	114.5	0.0
Other Current Liabilities
Current liability, related party	114.5	0.0
Other Liabilities, Noncurrent
Liability to unconsolidated affiliate (see Note 15)	$ 0.0	$ 98.8