Discontinued Operations - Major Components of Net Income (Loss) (Details) - PRB Transaction - Discontinued Operations - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Revenues:
Total revenues	$ 52	$ 148
Costs and expenses:
Depreciation, Depletion and amortization	145,913	145,913
Asset impairment	16,468	16,468
Other expenses	1,349	2,939
Other non-major expense (income) items, net	$ 189	$ 285