SUPPLEMENTAL QUARTERLY FINANCIAL DATA (Details) - USD ($)	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 [Line Items]
Operating revenue									$ 3,401,130,000	$ 3,346,496,000	$ 3,460,276,000
Operating Income (Loss)									519,008,000	554,058,000	739,106,000
Net Income (Loss) Attributable to Parent									210,708,000	235,622,000	175,194,000
Parent Company [Member]
Quarterly Financial Information [Line Items]
Operating revenue	$ 988,354,000	$ 627,007,000	$ 670,930,000	$ 1,114,839,000	$ 985,172,000	$ 651,464,000	$ 671,852,000	$ 1,038,008,000
Operating Income (Loss)	240,307,000	26,126,000	39,115,000	213,460,000	199,885,000	37,297,000	84,091,000	232,785,000
Net Income (Loss) Attributable to Parent	150,949,000	(39,443,000)	(32,952,000)	132,154,000	107,053,000	(21,970,000)	3,642,000	146,897,000	210,708,000	235,622,000	175,194,000
Subsidiaries [Member]
Quarterly Financial Information [Line Items]
Operating revenue	988,354,000	627,007,000	670,930,000	1,114,839,000	985,172,000	651,464,000	671,852,000	1,038,008,000	3,401,130,000	3,346,496,000	3,460,276,000
Operating Income (Loss)	241,955,000	26,721,000	39,780,000	214,159,000	193,432,000	46,147,000	81,701,000	235,856,000	522,615,000	557,136,000	740,595,000
Net Income (Loss) Attributable to Parent	$ 169,204,000	$ (15,257,000)	$ (8,325,000)	$ 147,302,000	$ 123,456,000	$ 3,891,000	$ 26,778,000	$ 163,037,000	$ 292,924,000	$ 317,162,000	$ 320,054,000