SUPPLEMENTAL QUARTERLY FINANCIAL DATA (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 [Line Items]
Operating revenues									$ 3,346,496	$ 3,460,276	$ 3,164,301
Operating income									554,058	739,106	765,474
Net income (loss)									235,622	175,194	312,899
Parent Company [Member]
Quarterly Financial Information [Line Items]
Operating revenues	$ 985,172	$ 651,464	$ 671,852	$ 1,038,008	$ 1,002,900	$ 660,377	$ 719,767	$ 1,077,232
Operating income	199,885	37,297	84,091	232,785	238,305	99,044	130,030	271,727
Net income (loss)	107,053	(21,970)	3,642	146,897	(467)	12,836	35,275	127,550	235,622	175,194	312,899
Subsidiaries [Member]
Quarterly Financial Information [Line Items]
Operating revenues	985,172	651,464	671,852	1,038,008	1,002,900	660,377	719,767	1,077,232	3,346,496	3,460,276	3,164,618
Operating income	193,432	46,147	81,701	235,856	248,995	96,369	126,800	268,431	557,136	740,595	770,552
Net income (loss)	$ 123,456	$ 3,891	$ 26,778	$ 163,037	$ 97,208	$ 29,100	$ 50,654	$ 143,092	$ 317,162	$ 320,054	$ 380,581