SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,988,512	$ 2,039,706	$ 5,856,458	$ 5,862,525
Depreciation and Amortization	(236,300)	(225,200)	(629,200)	(588,000)
Other Operating Expenses	(1,249,600)	(1,304,600)	(3,760,000)	(3,852,700)
Operating Income/(Loss)	502,603	509,869	1,467,310	1,421,827
Interest Expense	(108,719)	(99,865)	(319,477)	(298,568)
Other Income, Net	21,184	13,641	56,304	23,689
Net Income Attributable to Common Shareholders	260,370	265,319	750,577	713,123
Cash Flows Used for Investments in Plant			1,642,280	1,359,171
Assets	33,163,620		33,163,620		$ 32,053,173
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues	1,547,100	1,623,400	4,224,200	4,362,600
Depreciation and Amortization	(159,600)	(154,800)	(394,900)	(380,900)
Other Operating Expenses	(1,088,700)	(1,146,800)	(3,056,000)	(3,230,100)
Operating Income/(Loss)	298,800	321,800	773,300	751,600
Interest Expense	(51,300)	(49,000)	(149,000)	(144,600)
Other Income, Net	7,700	5,300	15,200	11,600
Net Income Attributable to Common Shareholders	157,400	170,100	393,400	381,300
Cash Flows Used for Investments in Plant			752,400	570,900
Assets	18,826,000		18,826,000		18,367,500
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues	109,200	99,200	698,800	622,300
Depreciation and Amortization	(15,200)	(15,200)	(54,800)	(47,900)
Other Operating Expenses	(95,500)	(87,800)	(535,200)	(462,400)
Operating Income/(Loss)	(1,500)	(3,800)	108,800	112,000
Interest Expense	(10,800)	(10,200)	(32,300)	(30,800)
Other Income, Net	300	600	800	500
Net Income Attributable to Common Shareholders	(6,200)	(7,000)	49,100	51,900
Cash Flows Used for Investments in Plant			209,800	170,300
Assets	3,432,600		3,432,600		3,303,800
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues	328,500	306,800	970,000	892,500
Depreciation and Amortization	(52,600)	(47,100)	(154,500)	(137,700)
Other Operating Expenses	(95,500)	(90,200)	(280,400)	(245,700)
Operating Income/(Loss)	180,400	169,500	535,100	509,100
Interest Expense	(29,200)	(26,900)	(86,100)	(82,200)
Other Income, Net	8,500	6,300	20,100	14,200
Net Income Attributable to Common Shareholders	99,000	88,400	289,600	266,600
Cash Flows Used for Investments in Plant			575,600	536,200
Assets	9,290,300		9,290,300		8,751,500
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating Revenues	224,200	211,500	677,500	636,800
Depreciation and Amortization	(9,500)	(8,600)	(26,700)	(23,100)
Other Operating Expenses	(190,000)	(179,300)	(602,400)	(564,700)
Operating Income/(Loss)	24,700	23,600	48,400	49,000
Interest Expense	(21,800)	(15,100)	(63,100)	(45,800)
Other Income, Net	267,500	256,900	853,900	781,400
Net Income Attributable to Common Shareholders	268,400	268,500	839,500	791,700
Cash Flows Used for Investments in Plant			104,500	81,800
Assets	14,939,400		14,939,400		14,493,100
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(220,500)	(201,200)	(714,000)	(651,700)
Depreciation and Amortization	600	500	1,700	1,600
Other Operating Expenses	220,100	199,500	714,000	650,200
Operating Income/(Loss)	200	(1,200)	1,700	100
Interest Expense	4,400	1,300	11,000	4,800
Other Income, Net	(262,800)	(255,500)	(833,700)	(784,000)
Net Income Attributable to Common Shareholders	(258,200)	$ (254,700)	(821,000)	(778,400)
Cash Flows Used for Investments in Plant			0	$ 0
Assets	$ (13,324,700)		$ (13,324,700)		$ (12,862,700)