SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,287,962	$ 2,105,135
Depreciation and Amortization	(249,500)	(210,800)
Other Operating Expenses	(1,596,000)	(1,393,300)
Operating Income/(Loss)	442,532	500,977
Interest Expense	(121,100)	103,400
Other Income, Net	33,789	21,619
Net Income Attributable to Common Shareholders	269,546	259,458
Cash Flows Used for Investments in Plant	607,334	523,560
Assets	37,039,983		$ 36,220,386
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues	1,754,600	1,401,100
Depreciation and Amortization	(144,400)	(129,800)
Other Operating Expenses	(1,443,500)	(1,048,900)
Operating Income/(Loss)	166,700	222,400
Interest Expense	(47,400)	48,200
Other Income, Net	19,600	12,000
Net Income Attributable to Common Shareholders	104,200	114,100
Cash Flows Used for Investments in Plant	236,000	236,200
Assets	21,167,900		19,250,400
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues	424,900	403,600
Depreciation and Amortization	(26,400)	(21,700)
Other Operating Expenses	(312,600)	(290,200)
Operating Income/(Loss)	85,900	91,700
Interest Expense	(11,100)	10,600
Other Income, Net	2,000	900
Net Income Attributable to Common Shareholders	57,800	50,800
Cash Flows Used for Investments in Plant	70,400	64,500
Assets	3,636,800		3,595,200
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues	305,000	316,900
Depreciation and Amortization	(56,600)	(50,600)
Other Operating Expenses	(83,200)	(90,200)
Operating Income/(Loss)	165,200	176,100
Interest Expense	(29,700)	28,100
Other Income, Net	8,000	5,100
Net Income Attributable to Common Shareholders	107,400	94,200
Cash Flows Used for Investments in Plant	239,200	192,600
Assets	9,833,200		9,401,200
Operating Segments | Water Distribution
Segment Reporting Information [Line Items]
Operating Revenues	44,600	0
Depreciation and Amortization	(10,700)	0
Other Operating Expenses	(23,900)	0
Operating Income/(Loss)	10,000	0
Interest Expense	(8,400)	0
Other Income, Net	(600)	0
Net Income Attributable to Common Shareholders	1,500	0
Cash Flows Used for Investments in Plant	19,000	0
Assets	2,487,900		2,470,000
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating Revenues	230,900	236,300
Depreciation and Amortization	(12,000)	(9,200)
Other Operating Expenses	(204,700)	(216,800)
Operating Income/(Loss)	14,200	10,300
Interest Expense	(31,900)	19,700
Other Income, Net	360,100	329,100
Net Income Attributable to Common Shareholders	346,000	322,200
Cash Flows Used for Investments in Plant	42,700	30,300
Assets	16,586,500		15,933,800
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(472,000)	(252,800)
Depreciation and Amortization	600	500
Other Operating Expenses	471,900	252,800
Operating Income/(Loss)	500	500
Interest Expense	7,400	(3,200)
Other Income, Net	(355,300)	(325,500)
Net Income Attributable to Common Shareholders	(347,400)	(321,800)
Cash Flows Used for Investments in Plant	0	$ 0
Assets	$ (16,672,300)		$ (14,430,200)