Segment Information (Schedule Of Operating Segment Results) (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment item	Sep. 30, 2020 USD ($)	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	$ 594,412,000	$ 382,261,000	1,402,345,000	$ 1,107,014,000
Adjusted EBITDA	131,595,000	17,426,000	229,018,000	19,609,000
Depreciation, depletion and amortization	30,760,000	32,630,000	84,441,000	94,105,000
Accretion on asset retirement obligations	5,437,000	4,947,000	16,311,000	14,939,000
Total assets	1,930,949,000	1,653,337,000	1,930,949,000	1,653,337,000	$ 1,722,472,000
Capital expenditures	64,089,000	57,100,000	212,046,000	205,661,000
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	295,291,000	168,054,000	693,522,000	489,660,000
Adjusted EBITDA	118,548,000	12,407,000	221,391,000	76,037,000
Depreciation, depletion and amortization	25,041,000	24,221,000	68,577,000	69,028,000
Accretion on asset retirement obligations	508,000	486,000	1,523,000	1,458,000
Total assets	965,252,000	759,202,000	965,252,000	759,202,000
Capital expenditures	61,504,000	57,424,000	204,347,000	193,586,000
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	299,096,000	213,299,000	707,394,000	597,887,000
Adjusted EBITDA	52,737,000	31,616,000	107,589,000	19,600,000
Depreciation, depletion and amortization	5,488,000	8,150,000	15,170,000	23,310,000
Accretion on asset retirement obligations	4,419,000	3,842,000	13,256,000	11,526,000
Total assets	193,060,000	216,708,000	193,060,000	216,708,000
Capital expenditures	507,000	568,000	1,370,000	9,381,000
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	25,000	908,000	1,429,000	19,467,000
Adjusted EBITDA	(39,690,000)	(26,597,000)	(99,962,000)	(76,028,000)
Depreciation, depletion and amortization	231,000	259,000	694,000	1,767,000
Accretion on asset retirement obligations	510,000	619,000	1,532,000	1,955,000
Total assets	772,637,000	677,427,000	772,637,000	677,427,000
Capital expenditures	$ 2,078,000	$ (892,000)	$ 6,329,000	$ 2,694,000