Other Assets and Liabilities - Other Current Assets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Other Assets and Liabilities [Line Items]
Note receivable		$ 0.0
Total	$ 115.1	135.7
Other current assets
Schedule of Other Assets and Liabilities [Line Items]
Short-term derivative assets (see Note 11)	48.7	61.9
Prepaid expenses	21.0	15.8
Environmental Credit Obligation Surplus (See Note 12)	20.2	10.3
Income and other tax receivables	7.8	24.3
Commodity investments	6.0	15.6
Note receivable	4.6
Other	6.8	7.8
Total	$ 115.1	$ 135.7