Note 17 - Segment Information - Segment Revenue (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020		Sep. 30, 2021	Sep. 30, 2020
Revenue	$ 292,016	$ 214,292		$ 925,998	$ 610,688
Revenue Benchmark [Member] | Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Revenue	43,947	45,818	[1]	133,863	94,647
Revenue Benchmark [Member] | Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Revenue	31,520			114,865	183,560
Revenue Benchmark [Member] | Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Revenue	$ 39,216	$ 36,888		110,253	$ 86,631
Revenue Benchmark [Member] | Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Customer D [Member]
Revenue				$ 140,314

[1] Revenues from these customers during the periods presented were less than 10% of the Company's total sales.