Discontinued Operations - Major Components of Net Income (Loss) (Details) - PRB Transaction - Discontinued Operations - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Revenues:
Total revenues	$ 4	$ 152
Costs and expenses:
Depreciation, Depletion and amortization	0	145,913
Accretion on asset retirement obligations	4,981	4,981
Asset impairment	694	17,162
Selling, general and administrative expenses	4,666	4,673
Other expenses	1,032	3,964
Other non-major expense (income) items, net	$ 147	$ 432