SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,415,792	$ 2,287,962
Depreciation and Amortization	(285,900)	(249,500)
Other Operating Expenses	(1,635,200)	(1,596,000)
Operating Income	494,700	442,532
Interest Expense	(131,734)	(121,129)
Other Income/(Loss), Net	30,985	33,789
Net Income Attributable to Common Shareholders	308,678	269,546
Cash Flows Used for Investments in Plant	674,694	607,334
Assets	38,941,007		$ 38,241,256
Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues
Other
Segment Reporting Information [Line Items]
Operating Revenues	258,100	230,900
Depreciation and Amortization	(13,600)	(12,000)
Other Operating Expenses	(225,500)	(204,700)
Operating Income	19,000	14,200
Interest Expense	(44,200)	(31,900)
Other Income/(Loss), Net	431,700	360,100
Net Income Attributable to Common Shareholders	407,500	346,000
Cash Flows Used for Investments in Plant	55,900	42,700
Assets	18,939,800		17,874,200
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(531,100)	(472,000)
Depreciation and Amortization	600	600
Other Operating Expenses	531,000	471,900
Operating Income	500	500
Interest Expense	12,600	7,400
Other Income/(Loss), Net	(427,600)	(355,300)
Net Income Attributable to Common Shareholders	(414,500)	(347,400)
Cash Flows Used for Investments in Plant	0	0
Assets	(18,326,600)		(17,464,900)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,833,900	1,754,600
Depreciation and Amortization	(179,200)	(144,400)
Other Operating Expenses	(1,475,600)	(1,443,500)
Operating Income	179,100	166,700
Interest Expense	(49,200)	(47,400)
Other Income/(Loss), Net	18,200	19,600
Net Income Attributable to Common Shareholders	120,100	104,200
Cash Flows Used for Investments in Plant	279,300	236,000
Assets	21,507,000		21,389,100
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	468,900	424,900
Depreciation and Amortization	(20,400)	(26,400)
Other Operating Expenses	(341,300)	(312,600)
Operating Income	107,200	85,900
Interest Expense	(11,700)	(11,100)
Other Income/(Loss), Net	200	2,000
Net Income Attributable to Common Shareholders	76,500	57,800
Cash Flows Used for Investments in Plant	87,400	70,400
Assets	4,019,100		3,904,900
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	340,500	305,000
Depreciation and Amortization	(61,500)	(56,600)
Other Operating Expenses	(98,800)	(83,200)
Operating Income	180,200	165,200
Interest Expense	(30,600)	(29,700)
Other Income/(Loss), Net	8,100	8,000
Net Income Attributable to Common Shareholders	118,200	107,400
Cash Flows Used for Investments in Plant	231,400	239,200
Assets	10,533,400		10,285,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	45,500	44,600
Depreciation and Amortization	(11,800)	(10,700)
Other Operating Expenses	(25,000)	(23,900)
Operating Income	8,700	10,000
Interest Expense	(8,600)	(8,400)
Other Income/(Loss), Net	400	(600)
Net Income Attributable to Common Shareholders	900	1,500
Cash Flows Used for Investments in Plant	20,700	$ 19,000
Assets	$ 2,268,300		$ 2,253,000