Segment and Geographic Information Segment Results (Details) - USD ($) $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 1,117.4	$ 1,106.4	$ 1,149.0	$ 1,250.6	$ 1,397.1	$ 1,412.6	$ 1,309.4	$ 1,462.7	$ 1,326.2	$ 4,252.6	$ 4,623.4	$ 5,581.8
Adjusted EBITDA									341.3	1,145.3	837.1	1,379.3
Additions to property, plant, equipment and mine development									32.8	166.6	285.4	301.0
Federal coal lease expenditures									0.5	0.0	0.0	0.5
Income from equity affiliates	$ (10.9)	$ (20.7)	$ (9.7)			$ (17.2)	$ (25.2)	$ (22.0)	(15.0)	(49.0)	(3.4)	(68.1)
Powder River Basin Mining
Segment Reporting Information [Line Items]
Revenues									394.3	1,178.7	1,228.7	1,424.8
Adjusted EBITDA									91.7	278.8	221.2	284.5
Additions to property, plant, equipment and mine development									19.3	32.6	42.8	81.0
Federal coal lease expenditures									0.0			0.0
Income from equity affiliates									0.0	0.0	0.0	0.0
Midwestern U.S. Mining
Segment Reporting Information [Line Items]
Revenues									193.2	592.3	669.7	801.0
Adjusted EBITDA									50.0	124.4	130.7	145.2
Additions to property, plant, equipment and mine development									2.8	21.7	35.9	46.6
Federal coal lease expenditures									0.0			0.0
Income from equity affiliates									0.0	0.0	0.0	0.0
Western U.S. Mining
Segment Reporting Information [Line Items]
Revenues									149.7	440.7	639.7	592.0
Adjusted EBITDA									50.0	131.8	230.7	145.4
Additions to property, plant, equipment and mine development									3.1	13.8	18.1	13.9
Federal coal lease expenditures									0.5			0.5
Income from equity affiliates									0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining
Segment Reporting Information [Line Items]
Revenues									328.9	1,221.0	1,033.1	1,553.0
Adjusted EBITDA									109.6	414.9	140.2	441.4
Additions to property, plant, equipment and mine development									5.2	56.0	143.4	88.7
Federal coal lease expenditures									0.0			0.0
Income from equity affiliates									0.0	0.0	0.0	0.0
Seaborne Thermal Mining
Segment Reporting Information [Line Items]
Revenues									224.8	772.5	971.7	1,099.2
Adjusted EBITDA									75.6	306.6	329.4	452.0
Additions to property, plant, equipment and mine development									2.3	39.2	42.1	66.6
Federal coal lease expenditures									0.0			0.0
Income from equity affiliates									0.0	0.0	0.0	0.0
Corporate and Other
Segment Reporting Information [Line Items]
Revenues									35.3	47.4	80.5	111.8
Adjusted EBITDA									(35.6)	(111.2)	(215.1)	(89.2)
Additions to property, plant, equipment and mine development									0.1	3.3	3.1	4.2
Federal coal lease expenditures									0.0			0.0
Income from equity affiliates									$ (15.0)	$ (49.0)	$ (3.4)	$ (68.1)