PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	$ 3,857,390	$ 3,608,347
Less accumulated depreciation, depletion and amortization	(2,040,468)	(1,909,669)
Total property, plant and equipment, net	1,816,922	1,698,678
Depreciation	270,900	256,900	$ 297,000
Mining Equipment
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	1,927,603	1,896,470
Land And Mineral Rights
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	499,950	458,440
Coal reserves not subject to depletion	29,900	37,400
Oil and gas mineral interests
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	740,635	647,864
Unproved properties	383,600	355,100
Buildings, Office Equipment And Improvements
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	300,436	282,902
Construction and mine development in progress
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	99,042	44,217
Mine development costs
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	289,724	$ 278,454
Mine development costs	$ 11,300