Segment Information - Assets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information, Additional Information [Abstract]
Assets	$ 4,860.9	$ 6,542.8
Property, plant, equipment and mine development, net	3,152.3	4,679.1
Operating lease right-of-use assets	44.9	82.4
Seaborne Mining
Segment Reporting Information, Additional Information [Abstract]
Assets	1,786.1	2,001.3
Property, plant, equipment and mine development, net	1,349.6	1,610.9
Operating lease right-of-use assets	22.4	32.1
U.S. Thermal Mining
Segment Reporting Information, Additional Information [Abstract]
Assets	1,430.5	3,044.8
Property, plant, equipment and mine development, net	1,275.2	2,776.9
Operating lease right-of-use assets	5.5	30.3
Corporate and Other
Segment Reporting Information, Additional Information [Abstract]
Assets	1,644.3	1,496.7
Property, plant, equipment and mine development, net	527.5	291.3
Operating lease right-of-use assets	$ 17.0	$ 20.0