SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,373,726	$ 2,415,792
Depreciation and Amortization	(286,000)	(285,900)
Other Operating Expenses	(1,548,600)	(1,635,200)
Operating Income/(Loss)	539,120	494,700
Interest Expense	(134,715)	(131,734)
Other Income, Net	24,104	30,985
Net Income/(Loss) Attributable to Common Shareholders	334,753	308,678
Cash Flows Used for Investments in Plant	725,520	628,129
Assets	41,648,986		$ 41,123,915
Other
Segment Reporting Information [Line Items]
Operating Revenues	294,600	258,100
Depreciation and Amortization	(21,300)	(13,600)
Other Operating Expenses	(257,000)	(225,500)
Operating Income/(Loss)	16,300	19,000
Interest Expense	(42,700)	(44,200)
Other Income, Net	468,100	431,700
Net Income/(Loss) Attributable to Common Shareholders	442,200	407,500
Cash Flows Used for Investments in Plant	59,300	55,900
Assets	20,860,800		20,469,600
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(591,400)	(531,100)
Depreciation and Amortization	600	600
Other Operating Expenses	591,400	531,000
Operating Income/(Loss)	600	500
Interest Expense	11,800	12,600
Other Income, Net	(463,200)	(427,600)
Net Income/(Loss) Attributable to Common Shareholders	(450,800)	(414,500)
Cash Flows Used for Investments in Plant	0	0
Assets	(20,284,300)		(19,488,800)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,805,300	1,833,900
Depreciation and Amortization	(167,300)	(179,200)
Other Operating Expenses	(1,439,600)	(1,475,600)
Operating Income/(Loss)	198,400	179,100
Interest Expense	(53,100)	(49,200)
Other Income, Net	12,500	18,200
Net Income/(Loss) Attributable to Common Shareholders	130,100	120,100
Cash Flows Used for Investments in Plant	315,800	279,300
Assets	23,135,900		22,541,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	448,600	468,900
Depreciation and Amortization	(19,100)	(20,400)
Other Operating Expenses	(311,600)	(341,300)
Operating Income/(Loss)	117,900	107,200
Interest Expense	(11,400)	(11,700)
Other Income, Net	1,400	200
Net Income/(Loss) Attributable to Common Shareholders	84,400	76,500
Cash Flows Used for Investments in Plant	99,100	87,400
Assets	4,430,400		4,345,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	369,700	340,500
Depreciation and Amortization	(67,500)	(61,500)
Other Operating Expenses	(106,800)	(98,800)
Operating Income/(Loss)	195,400	180,200
Interest Expense	(30,600)	(30,600)
Other Income, Net	5,200	8,100
Net Income/(Loss) Attributable to Common Shareholders	126,800	118,200
Cash Flows Used for Investments in Plant	232,800	184,800
Assets	11,137,500		10,904,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	46,900	45,500
Depreciation and Amortization	(11,400)	(11,800)
Other Operating Expenses	(25,000)	(25,000)
Operating Income/(Loss)	10,500	8,700
Interest Expense	(8,700)	(8,600)
Other Income, Net	100	400
Net Income/(Loss) Attributable to Common Shareholders	2,100	900
Cash Flows Used for Investments in Plant	18,500	$ 20,700
Assets	$ 2,368,700		$ 2,351,700