QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating Revenue	$ 344,000	$ 460,000	$ 354,000	$ 275,000	$ 304,000	$ 417,000	$ 352,000	$ 268,000	$ 1,432,618	$ 1,340,935	$ 1,234,995
Operating Income	36,000	126,000	83,000	43,000	44,000	138,000	107,000	37,000	288,144	326,326	243,131
Net Income	$ 11,000	$ 95,000	$ 58,000	$ 24,000	$ 13,000	$ 82,000	$ 61,000	$ 21,000	$ 188,323	$ 176,668	$ 124,438