QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating Revenue	$ 318,000	$ 441,000	$ 326,000	$ 333,000	$ 344,000	$ 460,000	$ 354,000	$ 275,000	$ 1,418,338	$ 1,432,618	$ 1,340,935
Operating Income	39,000	134,000	67,000	43,000	36,000	126,000	83,000	43,000	282,589	288,144	326,326
Net Income	$ 21,000	$ 98,000	$ 42,000	$ 26,000	$ 11,000	$ 95,000	$ 58,000	$ 24,000	$ 186,515	$ 188,323	$ 176,668