Segment Information (Schedule of Operating Segment Results) (Details) - USD ($) $ in Thousands	3 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	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 549,480	$ 619,467	$ 570,222	$ 555,183	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 2,294,352	$ 2,451,787	$ 2,324,623
Adjusted EBITDAR									363,167	437,778	419,697
Depreciation, depletion and amortization									112,055	119,563	122,464
Accretion on asset retirement obligations									20,548	27,970	30,209
Total Assets	1,867,756				1,887,060				1,867,756	1,887,060	1,979,632
Capital expenditures									266,356	95,272	59,205
Operating segments | PRB
Segment Reporting Information [Line Items]
Revenues									915,750	973,248	1,024,197
Adjusted EBITDAR									110,528	126,525	158,882
Depreciation, depletion and amortization									20,810	33,120	36,349
Accretion on asset retirement obligations									12,542	19,541	20,160
Total Assets	256,460				278,314				256,460	278,314	390,665
Capital expenditures									29,420	12,140	6,212
Operating segments | MET
Segment Reporting Information [Line Items]
Revenues									990,550	1,036,621	887,839
Adjusted EBITDAR									305,363	349,524	243,616
Depreciation, depletion and amortization									74,211	69,560	70,896
Accretion on asset retirement obligations									2,123	1,874	2,000
Total Assets	625,134				545,061				625,134	545,061	548,476
Capital expenditures									211,718	64,307	32,678
Operating segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues									377,202	428,884	396,504
Adjusted EBITDAR									41,495	68,620	102,006
Depreciation, depletion and amortization									14,414	14,699	13,588
Accretion on asset retirement obligations									2,413	2,261	2,161
Total Assets	105,411				125,333				105,411	125,333	134,397
Capital expenditures									20,088	11,999	11,901
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues									(10,850)	(13,034)	(16,083)
Adjusted EBITDAR									(94,219)	(106,891)	(84,807)
Depreciation, depletion and amortization									2,620	2,184	1,631
Accretion on asset retirement obligations									3,470	4,294	5,888
Total Assets	$ 880,751				$ 938,352				880,751	938,352	906,094
Capital expenditures									$ 5,130	$ 6,826	$ 8,414