QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating Revenue	$ 281,000	$ 394,000	$ 317,000	$ 243,000	$ 284,000	$ 409,000	$ 340,000	$ 273,000	$ 1,234,995	$ 1,306,544	$ 1,269,901
Operating Income	37,000	122,000	72,000	12,000	36,000	120,000	74,000	28,000	243,131	259,303	230,688
Net Income (Loss)	$ 12,000	$ 72,000	$ 41,000	$ (1,000)	$ 12,000	$ 69,000	$ 38,000	$ 9,000	$ 124,438	$ 127,794	$ 102,338