SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,105,135	$ 2,055,635
Depreciation and Amortization	(210,800)	(195,000)
Other Operating Expenses	(1,385,300)	(1,372,100)
Operating Income/(Loss)	509,019	488,536
Interest Expense	(103,429)	(98,212)
Other Income/(Loss), Net	13,577	2,011
Net Income Attributable to Common Shareholders	259,458	244,153
Cash Flows Used for Investments in Plant	523,560	431,472
Assets	32,355,176		$ 32,053,173
Operating Segments [Member] | Electric Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	1,401,100	1,436,100
Depreciation and Amortization	(129,800)	(127,700)
Other Operating Expenses	(1,041,900)	(1,088,900)
Operating Income/(Loss)	229,400	219,500
Interest Expense	(48,200)	(48,000)
Other Income/(Loss), Net	5,000	0
Net Income Attributable to Common Shareholders	114,100	108,400
Cash Flows Used for Investments in Plant	236,200	184,200
Assets	18,343,300		18,367,500
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	403,600	342,600
Depreciation and Amortization	(21,700)	(15,800)
Other Operating Expenses	(289,600)	(233,500)
Operating Income/(Loss)	92,300	93,300
Interest Expense	(10,600)	(10,100)
Other Income/(Loss), Net	300	(300)
Net Income Attributable to Common Shareholders	50,800	50,900
Cash Flows Used for Investments in Plant	64,500	52,100
Assets	3,332,700		3,303,800
Operating Segments [Member] | Electric Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	316,900	283,300
Depreciation and Amortization	(50,600)	(45,100)
Other Operating Expenses	(90,000)	(73,000)
Operating Income/(Loss)	176,300	165,200
Interest Expense	(28,100)	(28,000)
Other Income/(Loss), Net	4,900	2,600
Net Income Attributable to Common Shareholders	94,200	85,700
Cash Flows Used for Investments in Plant	192,600	172,400
Assets	9,015,600		8,751,500
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	236,300	214,200
Depreciation and Amortization	(9,200)	(6,900)
Other Operating Expenses	(216,600)	(197,300)
Operating Income/(Loss)	10,500	10,000
Interest Expense	(19,700)	(14,100)
Other Income/(Loss), Net	328,900	305,500
Net Income Attributable to Common Shareholders	322,200	302,500
Cash Flows Used for Investments in Plant	30,300	22,800
Assets	14,590,400		14,493,100
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(252,800)	(220,600)
Depreciation and Amortization	500	500
Other Operating Expenses	252,800	220,600
Operating Income/(Loss)	500	500
Interest Expense	3,200	2,000
Other Income/(Loss), Net	(325,500)	(305,800)
Net Income Attributable to Common Shareholders	(321,800)	(303,300)
Cash Flows Used for Investments in Plant	0	$ 0
Assets	$ (12,926,800)		$ (12,862,700)