SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,762,811	$ 1,767,184	$ 3,867,946	$ 3,822,819
Depreciation and Amortization	(182,100)	(167,800)	(392,900)	(362,800)
Other Operating Expenses	(1,125,000)	(1,176,000)	(2,510,300)	(2,548,000)
Operating Income	455,687	423,422	964,707	911,958
Interest Expense	(107,329)	(100,492)	(210,758)	(198,703)
Other Income/(Loss), Net	21,543	8,038	35,120	10,049
Net Income Attributable to Common Shareholders	230,749	203,649	490,207	447,803
Cash Flows Used for Investments in Plant			1,146,952	869,168
Assets	32,658,840		32,658,840		$ 32,053,173
Operating Segments [Member] | Electric Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	1,275,900	1,303,100	2,677,000	2,739,200
Depreciation and Amortization	(105,400)	(98,400)	(235,200)	(226,100)
Other Operating Expenses	(925,300)	(994,400)	(1,967,300)	(2,083,300)
Operating Income	245,200	210,300	474,500	429,800
Interest Expense	(49,400)	(47,600)	(97,600)	(95,600)
Other Income/(Loss), Net	2,500	6,300	7,400	6,300
Net Income Attributable to Common Shareholders	121,900	102,800	236,000	211,300
Cash Flows Used for Investments in Plant			515,000	362,000
Assets	18,663,700		18,663,700		18,367,500
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	186,000	180,400	589,600	523,000
Depreciation and Amortization	(17,900)	(16,800)	(39,600)	(32,700)
Other Operating Expenses	(150,000)	(141,100)	(439,600)	(374,500)
Operating Income	18,100	22,500	110,400	115,800
Interest Expense	(10,900)	(10,400)	(21,500)	(20,500)
Other Income/(Loss), Net	100	100	400	(200)
Net Income Attributable to Common Shareholders	4,500	8,000	55,300	58,900
Cash Flows Used for Investments in Plant			139,700	105,000
Assets	3,337,000		3,337,000		3,303,800
Operating Segments [Member] | Electric Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	324,600	302,500	641,500	585,800
Depreciation and Amortization	(51,400)	(45,600)	(102,000)	(90,700)
Other Operating Expenses	(94,900)	(82,500)	(184,800)	(155,500)
Operating Income	178,300	174,400	354,700	339,600
Interest Expense	(28,900)	(27,400)	(57,000)	(55,300)
Other Income/(Loss), Net	6,800	5,300	11,700	7,900
Net Income Attributable to Common Shareholders	96,400	92,500	190,600	178,200
Cash Flows Used for Investments in Plant			415,600	349,100
Assets	9,003,400		9,003,400		8,751,500
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	217,000	211,100	453,400	425,300
Depreciation and Amortization	(7,900)	(7,500)	(17,200)	(14,400)
Other Operating Expenses	(195,900)	(188,100)	(412,500)	(385,400)
Operating Income	13,200	15,500	23,700	25,500
Interest Expense	(21,600)	(16,600)	(41,300)	(30,700)
Other Income/(Loss), Net	257,600	219,000	586,500	524,400
Net Income Attributable to Common Shareholders	249,000	220,700	571,200	523,100
Cash Flows Used for Investments in Plant			76,700	53,100
Assets	14,657,100		14,657,100		14,493,100
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(240,700)	(229,900)	(493,600)	(450,500)
Depreciation and Amortization	500	500	1,100	1,100
Other Operating Expenses	241,100	230,100	493,900	450,700
Operating Income	900	700	1,400	1,300
Interest Expense	3,500	1,500	6,600	3,400
Other Income/(Loss), Net	(245,500)	(222,600)	(570,900)	(528,400)
Net Income Attributable to Common Shareholders	(241,100)	$ (220,400)	(562,900)	(523,700)
Cash Flows Used for Investments in Plant			0	$ 0
Assets	$ (13,002,400)		$ (13,002,400)		$ (12,862,700)