INCOME TAXES (Components of Net Deferred Income Tax Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
INCOME TAXES [Abstract]
Employee benefits	$ 21,874	$ 29,996
Incentive compensation	58,194	66,488
Asset retirement obligations & other reserves	58,327	55,033
State net operating losses	67,756	67,354
Other	25,362	44,169
Total gross deferred tax assets	231,513	263,040
Valuation allowance	(32,512)	(29,650)
Total net deferred tax asset	199,001	233,390
Property, plant & equipment	622,897	590,075
Goodwill/other intangible assets	252,209	238,712
Other	29,945	37,642
Total deferred tax liabilities	905,051	866,429
Net deferred tax liability	$ 706,050	$ 633,039