Revenue Recognition (Schedule of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 560,244	$ 613,538	$ 549,866	$ 600,975	$ 575,688		$ 2,451,787	$ 2,324,623
North America revenues
Disaggregation of Revenue [Line Items]
Revenues									445,661		1,340,887	1,622,161
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									130,027		1,110,900	702,462
Operating segments | PRB
Disaggregation of Revenue [Line Items]
Revenues									275,703		973,248	1,024,197
Operating segments | PRB | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									275,703		971,337	1,024,197
Operating segments | PRB | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									0		1,911	0
Operating segments | MET
Disaggregation of Revenue [Line Items]
Revenues									200,377		1,036,621	887,839
Operating segments | MET | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									91,086		160,969	312,874
Operating segments | MET | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									109,291		875,652	574,965
Operating segments | Other Thermal
Disaggregation of Revenue [Line Items]
Revenues									97,382		428,884	396,504
Operating segments | Other Thermal | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									76,646		195,547	269,007
Operating segments | Other Thermal | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									20,736		233,337	127,497
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues									2,226		13,034	16,083
Corporate, Other and Eliminations | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									2,226		13,034	16,083
Corporate, Other and Eliminations | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									$ 0		$ 0	$ 0
Predecessor
Disaggregation of Revenue [Line Items]
Revenues										$ 1,398,709
Predecessor | North America revenues
Disaggregation of Revenue [Line Items]
Revenues										1,175,326
Predecessor | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues										223,383
Predecessor | Operating segments | PRB
Disaggregation of Revenue [Line Items]
Revenues										726,747
Predecessor | Operating segments | PRB | North America revenues
Disaggregation of Revenue [Line Items]
Revenues										726,747
Predecessor | Operating segments | PRB | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues										0
Predecessor | Operating segments | MET
Disaggregation of Revenue [Line Items]
Revenues										437,069
Predecessor | Operating segments | MET | North America revenues
Disaggregation of Revenue [Line Items]
Revenues										215,585
Predecessor | Operating segments | MET | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues										221,484
Predecessor | Operating segments | Other Thermal
Disaggregation of Revenue [Line Items]
Revenues										213,052
Predecessor | Operating segments | Other Thermal | North America revenues
Disaggregation of Revenue [Line Items]
Revenues										211,153
Predecessor | Operating segments | Other Thermal | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues										1,899
Predecessor | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues										21,841
Predecessor | Corporate, Other and Eliminations | North America revenues
Disaggregation of Revenue [Line Items]
Revenues										21,841
Predecessor | Corporate, Other and Eliminations | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues										$ 0