Note 15 - Segment Information - Segment Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue	$ 121,296	$ 371,182	$ 396,396	$ 728,165
Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Customer B [Member]
Revenue	30,032	139,811	134,386	262,930
Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Customer C [Member]
Revenue	$ 14,060	53,511	49,743	95,377
Pennsylvania Mining Complex [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Customer A [Member]
Revenue		$ 68,370	$ 48,829	$ 130,241