SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,122,538	$ 1,953,128	$ 4,948,378	$ 4,326,854
Depreciation and Amortization	(280,200)	(263,900)	(659,000)	(549,900)
Other Operating Expenses	(1,390,700)	(1,255,600)	(3,252,200)	(2,804,200)
Operating Income, Net	451,605	433,655	1,037,173	972,775
Interest Expense	(145,435)	(134,285)	(283,201)	(269,000)
Other Income, Net	46,619	30,243	80,820	54,347
Net Income Attributable to Common Shareholders	264,520	252,232	630,663	586,984
Cash Flows Used for Investments in Plant			1,423,223	1,400,198
Assets	47,234,640		47,234,640		$ 46,099,598
Other
Segment Reporting Information [Line Items]
Operating Revenues	330,000	283,800	673,000	578,500
Depreciation and Amortization	(28,400)	(22,700)	(55,700)	(44,000)
Other Operating Expenses	(285,700)	(243,100)	(585,200)	(500,000)
Operating Income, Net	15,900	18,000	32,100	34,500
Interest Expense	(41,800)	(37,600)	(83,400)	(80,300)
Other Income, Net	309,200	303,200	733,400	686,900
Net Income Attributable to Common Shareholders	282,700	282,800	677,600	639,200
Cash Flows Used for Investments in Plant			109,900	124,600
Assets	21,669,000		21,669,000		22,089,400
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(659,800)	(590,300)	(1,304,900)	(1,181,700)
Depreciation and Amortization	1,100	300	2,100	900
Other Operating Expenses	660,300	593,500	1,305,200	1,184,800
Operating Income, Net	1,600	3,500	2,400	4,000
Interest Expense	12,800	8,800	24,400	20,500
Other Income, Net	(304,800)	(300,400)	(725,800)	(679,100)
Net Income Attributable to Common Shareholders	(290,400)	(288,100)	(699,000)	(654,600)
Cash Flows Used for Investments in Plant			0	0
Assets	(21,641,100)		(21,641,100)		(21,492,400)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,676,500	1,616,200	3,576,900	3,421,600
Depreciation and Amortization	(132,300)	(140,700)	(354,300)	(308,100)
Other Operating Expenses	(1,360,300)	(1,293,300)	(2,890,800)	(2,732,900)
Operating Income, Net	183,900	182,200	331,800	380,600
Interest Expense	(61,100)	(54,300)	(114,400)	(107,400)
Other Income, Net	29,900	16,400	50,600	28,900
Net Income Attributable to Common Shareholders	121,600	115,000	214,900	245,100
Cash Flows Used for Investments in Plant			510,400	563,200
Assets	25,904,600		25,904,600		24,981,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	311,800	211,200	1,092,300	659,800
Depreciation and Amortization	(34,600)	(21,200)	(80,300)	(40,300)
Other Operating Expenses	(257,700)	(176,100)	(793,600)	(487,700)
Operating Income, Net	19,500	13,900	218,400	131,800
Interest Expense	(14,600)	(10,700)	(28,500)	(22,000)
Other Income, Net	4,600	400	8,500	1,800
Net Income Attributable to Common Shareholders	4,100	2,600	151,600	88,600
Cash Flows Used for Investments in Plant			305,900	205,600
Assets	6,593,200		6,593,200		6,450,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	411,200	376,700	811,900	746,300
Depreciation and Amortization	(74,500)	(68,800)	(148,000)	(136,200)
Other Operating Expenses	(121,900)	(111,200)	(237,300)	(218,000)
Operating Income, Net	214,800	196,700	426,600	392,100
Interest Expense	(32,600)	(32,100)	(65,300)	(62,700)
Other Income, Net	6,800	10,600	12,200	15,700
Net Income Attributable to Common Shareholders	137,600	129,500	273,000	256,200
Cash Flows Used for Investments in Plant			443,200	460,800
Assets	12,289,300		12,289,300		11,695,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	52,800	55,500	99,200	102,400
Depreciation and Amortization	(11,500)	(10,800)	(22,800)	(22,200)
Other Operating Expenses	(25,400)	(25,400)	(50,500)	(50,400)
Operating Income, Net	15,900	19,300	25,900	29,800
Interest Expense	(8,100)	(8,400)	(16,000)	(17,100)
Other Income, Net	900	0	1,900	100
Net Income Attributable to Common Shareholders	8,900	$ 10,400	12,600	12,500
Cash Flows Used for Investments in Plant			53,800	$ 46,000
Assets	$ 2,419,600		$ 2,419,600		$ 2,375,200