Business Segment Information - Segment Profit or Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Sales and other operating revenue	$ 404.3	$ 364.5	$ 1,203.1	$ 1,082.0
Adjusted EBITDA	66.7	66.0	197.1	197.3
Depreciation and amortization expense	35.6	35.4	109.8	100.3
Capital expenditures	28.4	27.0	81.5	70.7
Domestic Coke
Segment Reporting Information [Line Items]
Adjusted EBITDA	59.8	49.1	174.6	156.3
Depreciation and amortization expense	28.9	28.7	90.1	79.5
Capital expenditures	28.0	25.1	78.4	67.1
Brazil Coke
Segment Reporting Information [Line Items]
Adjusted EBITDA	3.9	4.5	12.7	14.0
Depreciation and amortization expense	0.2	0.2	0.5	0.5
Logistics
Segment Reporting Information [Line Items]
Adjusted EBITDA	9.6	21.0	34.1	54.3
Depreciation and amortization expense	6.1	6.2	18.2	19.2
Capital expenditures	0.4	1.9	3.1	3.5
Corporate and Other
Segment Reporting Information [Line Items]
Adjusted EBITDA	(6.6)	(8.6)	(24.3)	(27.3)
Depreciation and amortization expense	0.4	0.3	1.0	1.1
Capital expenditures	0.0	0.0	0.0	0.1
Coal Mining
Segment Reporting Information [Line Items]
Adjusted EBITDA	(2.0)	(2.9)	(5.8)	(7.6)
Operating Segments | Domestic Coke
Segment Reporting Information [Line Items]
Sales and other operating revenue	378.5	326.8	1,115.8	973.6
Operating Segments | Brazil Coke
Segment Reporting Information [Line Items]
Sales and other operating revenue	9.6	9.7	29.3	30.0
Operating Segments | Logistics
Segment Reporting Information [Line Items]
Sales and other operating revenue	16.2	28.0	58.0	78.4
Elimination of intersegment sales
Segment Reporting Information [Line Items]
Sales and other operating revenue	(6.1)	(5.7)	(19.3)	(16.6)
Elimination of intersegment sales | Logistics
Segment Reporting Information [Line Items]
Sales and other operating revenue	$ 6.1	$ 5.7	$ 19.3	$ 16.6