INCOME TAXES (Components of Net Deferred Income Tax Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
INCOME TAXES [Abstract]
Employee benefits	$ 29,996	$ 24,407
Incentive compensation	66,488	62,829
Asset retirement obligations & other reserves	55,033	55,822
State net operating losses	67,354	68,436
Other	44,169	31,294
Total gross deferred tax assets	263,040	242,788
Valuation allowance	(29,650)	(29,680)
Total net deferred tax asset	233,390	213,108
Property, plant & equipment	590,075	510,604
Goodwill/other intangible assets	238,712	233,471
Other	37,642	36,316
Total deferred tax liabilities	866,429	780,391
Net deferred tax liability	$ 633,039	$ 567,283