SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,271,425	$ 1,988,512	$ 6,413,243	$ 5,856,458
Depreciation and Amortization	(301,400)	(236,300)	(786,200)	(629,200)
Other Operating Expenses	(1,504,000)	(1,256,900)	(4,327,100)	(3,782,900)
Operating Income/(Loss)	466,021	495,251	1,299,907	1,444,436
Interest Expense	(125,200)	(108,700)	(372,700)	(319,500)
Other Income/(Loss), Net	16,718	28,536	100,656	79,178
Net Income Attributable to Common Shareholders	289,380	260,370	801,693	750,577
Cash Flows Used for Investments in Plant			1,885,081	1,642,280
Assets	37,515,669		37,515,669		$ 36,220,386
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues	2,027,400	1,547,100	5,321,800	4,224,200
Depreciation and Amortization	(206,100)	(159,600)	(486,000)	(394,900)
Other Operating Expenses	(1,562,200)	(1,095,200)	(4,238,600)	(3,076,100)
Operating Income/(Loss)	259,100	292,300	597,200	753,200
Interest Expense	(52,400)	(51,300)	(152,000)	(149,000)
Other Income/(Loss), Net	32,200	14,200	70,900	35,300
Net Income Attributable to Common Shareholders	173,800	157,400	379,300	393,400
Cash Flows Used for Investments in Plant			717,400	752,400
Assets	21,189,000		21,189,000		19,250,400
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues	109,000	109,200	740,500	698,800
Depreciation and Amortization	(13,900)	(15,200)	(59,600)	(54,800)
Other Operating Expenses	(102,400)	(96,200)	(585,700)	(537,300)
Operating Income/(Loss)	(7,300)	(2,200)	95,200	106,700
Interest Expense	(11,300)	(10,800)	(33,700)	(32,300)
Other Income/(Loss), Net	1,500	1,000	5,100	2,900
Net Income Attributable to Common Shareholders	(12,600)	(6,200)	50,200	49,100
Cash Flows Used for Investments in Plant			245,500	209,800
Assets	3,747,400		3,747,400		3,595,200
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues	330,600	328,500	953,000	970,000
Depreciation and Amortization	(58,300)	(52,600)	(171,800)	(154,500)
Other Operating Expenses	(96,700)	(95,500)	(268,700)	(280,600)
Operating Income/(Loss)	175,600	180,400	512,500	534,900
Interest Expense	(30,300)	(29,200)	(89,900)	(86,100)
Other Income/(Loss), Net	9,000	8,500	26,700	20,300
Net Income Attributable to Common Shareholders	109,500	99,000	329,600	289,600
Cash Flows Used for Investments in Plant			735,800	575,600
Assets	10,077,500		10,077,500		9,401,200
Operating Segments | Water Distribution
Segment Reporting Information [Line Items]
Operating Revenues	63,500	0	161,500	0
Depreciation and Amortization	(11,700)	0	(34,600)	0
Other Operating Expenses	(25,500)	0	(73,900)	0
Operating Income/(Loss)	26,300	0	53,000	0
Interest Expense	(8,500)	0	(25,500)	0
Other Income/(Loss), Net	700	0	(400)	0
Net Income Attributable to Common Shareholders	17,600	0	26,300	0
Cash Flows Used for Investments in Plant			68,100	0
Assets	2,240,300		2,240,300		2,470,000
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating Revenues	224,600	224,200	692,500	677,500
Depreciation and Amortization	(12,000)	(9,500)	(35,900)	(26,700)
Other Operating Expenses	(200,900)	(190,100)	(617,200)	(602,900)
Operating Income/(Loss)	11,700	24,600	39,400	47,900
Interest Expense	(30,500)	(21,800)	(94,800)	(63,100)
Other Income/(Loss), Net	251,700	267,600	913,800	854,400
Net Income Attributable to Common Shareholders	271,100	268,400	905,900	839,500
Cash Flows Used for Investments in Plant			118,300	104,500
Assets	16,979,400		16,979,400		15,933,800
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(483,700)	(220,500)	(1,456,100)	(714,000)
Depreciation and Amortization	600	600	1,700	1,700
Other Operating Expenses	483,700	220,100	1,457,000	714,000
Operating Income/(Loss)	600	200	2,600	1,700
Interest Expense	7,800	4,400	23,200	11,000
Other Income/(Loss), Net	(278,400)	(262,800)	(915,400)	(833,700)
Net Income Attributable to Common Shareholders	(270,000)	$ (258,200)	(889,600)	(821,000)
Cash Flows Used for Investments in Plant			0	$ 0
Assets	$ (16,717,900)		$ (16,717,900)		$ (14,430,200)