SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,825,840	$ 2,373,726
Depreciation and Amortization	(378,700)	(286,000)
Other Operating Expenses	(1,861,500)	(1,548,600)
Operating Income/(Loss)	585,568	539,120
Interest Expense	(137,766)	(134,715)
Other Income, Net	34,201	24,104
Net Income/(Loss) Attributable to Common Shareholders	366,143	334,753
Cash Flows Used for Investments in Plant	688,983	725,520
Assets	46,518,215		$ 46,099,598
Other
Segment Reporting Information [Line Items]
Operating Revenues	343,000	294,600
Depreciation and Amortization	(27,300)	(21,300)
Other Operating Expenses	(299,500)	(257,000)
Operating Income/(Loss)	16,200	16,300
Interest Expense	(41,600)	(42,700)
Other Income, Net	424,100	383,700
Net Income/(Loss) Attributable to Common Shareholders	394,900	356,300
Cash Flows Used for Investments in Plant	47,800	59,300
Assets	21,468,200		22,089,400
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(645,000)	(591,400)
Depreciation and Amortization	1,000	600
Other Operating Expenses	644,800	591,400
Operating Income/(Loss)	800	600
Interest Expense	11,600	11,800
Other Income, Net	(421,000)	(378,800)
Net Income/(Loss) Attributable to Common Shareholders	(408,600)	(366,400)
Cash Flows Used for Investments in Plant	0	0
Assets	(20,664,500)		(21,492,400)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,900,300	1,805,300
Depreciation and Amortization	(221,900)	(167,300)
Other Operating Expenses	(1,530,500)	(1,439,600)
Operating Income/(Loss)	147,900	198,400
Interest Expense	(53,300)	(53,100)
Other Income, Net	20,700	12,500
Net Income/(Loss) Attributable to Common Shareholders	93,200	130,100
Cash Flows Used for Investments in Plant	256,600	315,800
Assets	25,033,900		24,981,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	780,500	448,600
Depreciation and Amortization	(45,700)	(19,100)
Other Operating Expenses	(535,800)	(311,600)
Operating Income/(Loss)	199,000	117,900
Interest Expense	(13,900)	(11,400)
Other Income, Net	3,900	1,400
Net Income/(Loss) Attributable to Common Shareholders	147,600	85,900
Cash Flows Used for Investments in Plant	131,800	99,100
Assets	6,576,500		6,450,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	400,600	369,700
Depreciation and Amortization	(73,500)	(67,500)
Other Operating Expenses	(115,400)	(106,800)
Operating Income/(Loss)	211,700	195,400
Interest Expense	(32,700)	(30,600)
Other Income, Net	5,500	5,200
Net Income/(Loss) Attributable to Common Shareholders	135,400	126,800
Cash Flows Used for Investments in Plant	225,200	232,800
Assets	11,713,100		11,695,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	46,400	46,900
Depreciation and Amortization	(11,300)	(11,400)
Other Operating Expenses	(25,100)	(25,000)
Operating Income/(Loss)	10,000	10,500
Interest Expense	(7,900)	(8,700)
Other Income, Net	1,000	100
Net Income/(Loss) Attributable to Common Shareholders	3,600	2,100
Cash Flows Used for Investments in Plant	27,600	$ 18,500
Assets	$ 2,391,000		$ 2,375,200