Segment Information (Schedule Of Operating Segment Results) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment item	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) segment
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	$ 450,389,000	$ 319,521,000	807,932,000	$ 724,753,000
Adjusted EBITDA	66,526,000	(10,732,000)	97,423,000	2,183,000
Depreciation, depletion and amortization	27,884,000	30,167,000	53,681,000	61,475,000
Accretion on asset retirement obligations	5,437,000	4,986,000	10,874,000	9,992,000
Total assets	1,794,946,000	1,802,389,000	1,794,946,000	1,802,389,000	$ 1,722,472,000
Capital expenditures	71,199,000	60,872,000	147,957,000	148,561,000
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	219,448,000	138,951,000	398,231,000	321,605,000
Adjusted EBITDA	61,246,000	20,910,000	102,843,000	63,630,000
Depreciation, depletion and amortization	22,654,000	22,289,000	43,536,000	44,807,000
Accretion on asset retirement obligations	507,000	486,000	1,016,000	972,000
Total assets	941,843,000	740,451,000	941,843,000	740,451,000
Capital expenditures	66,823,000	57,514,000	142,843,000	136,162,000
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	230,759,000	174,393,000	408,297,000	384,589,000
Adjusted EBITDA	41,772,000	(10,114,000)	54,853,000	(12,016,000)
Depreciation, depletion and amortization	4,993,000	7,616,000	9,682,000	15,161,000
Accretion on asset retirement obligations	4,419,000	3,842,000	8,837,000	7,685,000
Total assets	191,164,000	356,228,000	191,164,000	356,228,000
Capital expenditures	575,000	2,100,000	864,000	8,813,000
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	182,000	6,177,000	1,404,000	18,559,000
Adjusted EBITDA	(36,492,000)	(21,528,000)	(60,273,000)	(49,431,000)
Depreciation, depletion and amortization	237,000	262,000	463,000	1,507,000
Accretion on asset retirement obligations	511,000	658,000	1,021,000	1,335,000
Total assets	661,939,000	705,710,000	661,939,000	705,710,000
Capital expenditures	$ 3,801,000	$ 1,258,000	$ 4,250,000	$ 3,586,000