INCOME TAXES (Components of Net Deferred Income Tax Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
INCOME TAXES [Abstract]
Employee benefits	$ 29,547	$ 85,123
Asset retirement obligations & other reserves	47,116	63,617
Deferred compensation	59,010	103,947
State net operating losses	73,083	54,498
Federal credit carryforwards	51,284	18,139
Other	37,518	44,843
Total gross deferred tax assets	297,558	370,167
Valuation allowance	(29,723)	(44,237)
Total net deferred tax assets	267,835	325,930
Property, plant & equipment	490,459	664,763
Goodwill/other intangible assets	216,039	327,666
Other	25,418	36,355
Total deferred tax liabilities	731,916	1,028,784
Net deferred tax liability	$ 464,081	$ 702,854