INCOME TAXES (Components of Net Deferred Income Tax Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
INCOME TAXES [Abstract]
Employee benefits	$ 24,407	$ 29,547
Incentive compensation	62,829	59,010
Asset retirement obligations & other reserves	55,822	47,116
State net operating losses	68,436	73,083
Federal credit carryforwards	0	51,284
Other	31,294	37,518
Total gross deferred tax assets	242,788	297,558
Valuation allowance	(29,680)	(29,723)
Total net deferred tax asset	213,108	267,835
Property, plant & equipment	510,604	490,459
Goodwill/other intangible assets	233,471	216,039
Other	36,316	25,418
Total deferred tax liabilities	780,391	731,916
Net deferred tax liability	$ 567,283	$ 464,081