Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment	2
Segment Reporting Information [Line Items]
Revenues	$ 575,295	$ 600,975
Adjusted EBITDA	104,913	121,219
Depreciation, depletion and amortization	29,703	31,921
Accretion on asset retirement obligations	6,992	7,623
Assets	1,976,678	2,144,916	$ 1,979,632
Capital expenditures	9,453	5,950
PRB
Segment Reporting Information [Line Items]
Revenues	245,428	273,428
MET
Segment Reporting Information [Line Items]
Revenues	238,347	225,582
Other Thermal
Segment Reporting Information [Line Items]
Revenues	91,520	101,906
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	245,428	273,428
Adjusted EBITDA	27,502	48,006
Depreciation, depletion and amortization	8,423	9,510
Accretion on asset retirement obligations	4,885	5,040
Assets	383,823	433,370
Capital expenditures	698	128
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	238,347	225,582
Adjusted EBITDA	83,742	68,310
Depreciation, depletion and amortization	16,986	18,764
Accretion on asset retirement obligations	469	528
Assets	548,804	575,789
Capital expenditures	5,829	4,610
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	91,520	101,906
Adjusted EBITDA	15,669	27,242
Depreciation, depletion and amortization	3,835	3,200
Accretion on asset retirement obligations	565	540
Assets	130,132	128,529
Capital expenditures	1,206	741
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	0	59
Adjusted EBITDA	(22,000)	(22,339)
Depreciation, depletion and amortization	459	447
Accretion on asset retirement obligations	1,073	1,515
Assets	913,919	1,007,228
Capital expenditures	$ 1,720	$ 471