Segment Information (Schedule Of Operating Segment Results) (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) item segment	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2	3
Number of remaining active thermal mines | item	3
Number of distinct lines of business	$ 2
Revenues	2,208,042,000	$ 1,467,592,000	$ 2,294,352,000
Adjusted EBITDA	533,430,000	23,743,000	363,167,000
Depreciation, depletion and amortization	120,327,000	121,552,000	111,621,000
Accretion on asset retirement obligations	21,748,000	19,887,000	20,548,000
Total assets	2,117,160,000	1,722,472,000	1,867,756,000
Capital expenditures	245,440,000	285,821,000	266,356,000
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	1,149,133,000	641,536,000	990,550,000
Adjusted EBITDA	442,830,000	91,322,000	305,363,000
Depreciation, depletion and amortization	99,171,000	91,202,000	74,211,000
Accretion on asset retirement obligations	2,030,000	1,943,000	2,123,000
Total assets	964,761,000	811,605,000	625,134,000
Capital expenditures	227,802,000	269,273,000	211,718,000
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	1,057,480,000	801,632,000	1,292,952,000
Adjusted EBITDA	175,709,000	34,035,000	152,023,000
Depreciation, depletion and amortization	20,231,000	28,351,000	35,224,000
Accretion on asset retirement obligations	17,675,000	15,368,000	14,955,000
Total assets	205,147,000	196,336,000	361,871,000
Capital expenditures	5,949,000	10,719,000	49,508,000
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	1,429,000	24,424,000	10,850,000
Adjusted EBITDA	(85,109,000)	(101,614,000)	(94,219,000)
Depreciation, depletion and amortization	925,000	1,999,000	2,186,000
Accretion on asset retirement obligations	2,043,000	2,576,000	3,470,000
Total assets	947,252,000	714,531,000	880,751,000
Capital expenditures	$ 11,689,000	$ 5,829,000	$ 5,130,000