Supplemental Coal Data (unaudited) (Schedule of Proven and Probable Reserves) (Details) - Coal segment - T T in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Proved Developed and Undeveloped Reserve (Mass) Rollforward [Roll Forward]
Consolidated recoverable coal reserves at beginning of period	2,261.0	2,298.0	2,361.0	3,047.0	3,238.0
Purchased reserves	0.0	0.0	0.0	0.0	24.0
Reserves sold in place	0.0	0.0	(16.0)	(601.0)	(43.0)
Production	(27.0)	(28.0)	(26.0)	(26.0)	(29.0)
Revisions and other changes	(8.0)	(9.0)	(21.0)	(59.0)	(143.0)
Consolidated recoverable coal reserves at end of period	2,226.0	2,261.0	2,298.0	2,361.0	3,047.0
Consolidation Coal Company
Proved Developed and Undeveloped Reserve (Mass) Rollforward [Roll Forward]
Consolidated recoverable coal reserves at end of period	143.3