SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,432,794	$ 2,343,642	$ 7,381,172	$ 6,670,497
Depreciation and Amortization	(322,100)	(302,000)	(981,100)	(851,900)
Other Operating Expenses	(1,627,400)	(1,480,400)	(4,879,600)	(4,284,600)
Operating Income/(Loss)	483,331	561,230	1,520,503	1,534,006
Interest Expense	(147,962)	(134,066)	(431,162)	(403,067)
Other Income, Net	43,768	29,218	124,588	83,565
Net Income/(Loss) Attributable to Common Shareholders	283,166	346,260	913,829	933,244
Cash Flows Used for Investments in Plant			2,211,136	2,101,564
Assets	48,014,055		48,014,055		$ 46,099,598
Other
Segment Reporting Information [Line Items]
Operating Revenues	326,100	322,300	999,100	900,700
Depreciation and Amortization	(28,000)	(24,100)	(83,700)	(68,100)
Other Operating Expenses	(275,200)	(271,500)	(860,300)	(771,400)
Operating Income/(Loss)	22,900	26,700	55,100	61,200
Interest Expense	(42,500)	(38,700)	(125,900)	(119,000)
Other Income, Net	325,900	401,000	1,059,200	1,087,900
Net Income/(Loss) Attributable to Common Shareholders	304,900	396,600	982,500	1,035,600
Cash Flows Used for Investments in Plant			157,600	181,200
Assets	21,624,500		21,624,500		22,089,400
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(614,100)	(602,600)	(1,918,900)	(1,784,300)
Depreciation and Amortization	1,100	300	3,100	1,200
Other Operating Expenses	613,700	603,100	1,918,900	1,787,900
Operating Income/(Loss)	700	800	3,100	4,800
Interest Expense	12,700	7,100	37,000	27,600
Other Income, Net	(320,400)	(397,000)	(1,046,200)	(1,076,100)
Net Income/(Loss) Attributable to Common Shareholders	(307,000)	(389,100)	(1,006,100)	(1,043,700)
Cash Flows Used for Investments in Plant			0	0
Assets	(20,648,600)		(20,648,600)		(21,492,400)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	2,055,200	2,060,300	5,632,100	5,481,900
Depreciation and Amortization	(179,800)	(182,000)	(534,100)	(490,000)
Other Operating Expenses	(1,640,500)	(1,577,000)	(4,531,300)	(4,309,900)
Operating Income/(Loss)	234,900	301,300	566,700	682,000
Interest Expense	(61,000)	(53,300)	(175,400)	(160,700)
Other Income, Net	25,400	16,000	76,000	44,900
Net Income/(Loss) Attributable to Common Shareholders	150,400	205,500	365,400	450,600
Cash Flows Used for Investments in Plant			764,900	781,200
Assets	25,604,200		25,604,200		24,981,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	185,500	110,200	1,277,800	770,000
Depreciation and Amortization	(28,500)	(14,800)	(108,700)	(55,100)
Other Operating Expenses	(176,900)	(104,200)	(970,600)	(592,000)
Operating Income/(Loss)	(19,900)	(8,800)	198,500	122,900
Interest Expense	(15,700)	(10,100)	(44,200)	(32,100)
Other Income, Net	6,700	700	15,200	2,500
Net Income/(Loss) Attributable to Common Shareholders	(22,000)	(15,400)	129,600	73,300
Cash Flows Used for Investments in Plant			506,400	322,800
Assets	6,865,600		6,865,600		6,450,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	418,300	389,300	1,230,100	1,135,600
Depreciation and Amortization	(75,400)	(70,600)	(223,400)	(206,800)
Other Operating Expenses	(122,400)	(120,600)	(359,600)	(338,500)
Operating Income/(Loss)	220,500	198,100	647,100	590,300
Interest Expense	(33,500)	(31,100)	(98,700)	(93,800)
Other Income, Net	5,000	6,900	17,300	22,700
Net Income/(Loss) Attributable to Common Shareholders	139,400	125,600	412,400	381,800
Cash Flows Used for Investments in Plant			691,000	732,400
Assets	12,123,500		12,123,500		11,695,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	61,800	64,100	161,000	166,600
Depreciation and Amortization	(11,500)	(10,800)	(34,300)	(33,100)
Other Operating Expenses	(26,100)	(10,200)	(76,700)	(60,700)
Operating Income/(Loss)	24,200	43,100	50,000	72,800
Interest Expense	(8,000)	(8,000)	(24,000)	(25,100)
Other Income, Net	1,200	1,600	3,100	1,700
Net Income/(Loss) Attributable to Common Shareholders	17,500	$ 23,100	30,000	35,600
Cash Flows Used for Investments in Plant			91,200	$ 84,000
Assets	$ 2,444,900		$ 2,444,900		$ 2,375,200