Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Inventory [Line Items]
Inventory valuation reserves	$ 0.8		$ 0.8		$ 2.4
Excess of replacement costs (FIFO) over carrying value (LIFO)	7.4		7.4		9.0
Cost of goods sold
Inventory [Line Items]
Inventory valuation gains (losses)	0.3	$ (10.8)	0.7	$ (13.5)
Liquidation gain (loss) on refinery inventory	1.0	$ 0.2	0.2	$ 0.2
LIFO inventory
Inventory [Line Items]
Inventory valuation reserves	$ 0.5		$ 0.5		$ 1.5