SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,039,706	$ 1,933,105	$ 5,862,525	$ 6,263,597
Depreciation and Amortization	(225,200)	(151,000)	(588,000)	(538,000)
Other Operating Expenses	(1,304,600)	(1,312,900)	(3,852,700)	(4,346,900)
Operating Income/(Loss)	509,869	469,238	1,421,827	1,378,728
Interest Expense	(99,865)	(92,534)	(298,568)	(279,635)
Other Income/(Loss), Net	13,641	5,241	23,689	23,866
Net Income Attributable to Common Shareholders	265,319	235,920	713,123	696,680
Cash Flows Used for Investments in Plant			1,359,171	1,177,285
Assets	31,082,337		31,082,337		$ 30,580,309
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(201,200)	(198,800)	(651,700)	(664,900)
Depreciation and Amortization	500	500	1,600	1,600
Other Operating Expenses	199,500	198,500	650,200	665,200
Operating Income/(Loss)	(1,200)	200	100	1,900
Interest Expense	1,300	1,200	4,800	3,400
Other Income/(Loss), Net	(255,500)	(242,200)	(784,000)	(774,500)
Net Income Attributable to Common Shareholders	(254,700)	(240,800)	(778,400)	(769,200)
Cash Flows Used for Investments in Plant			0	0
Assets	(12,286,800)		(12,286,800)		(11,781,500)
Operating Segments [Member] | Electric Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	1,623,400	1,543,700	4,362,600	4,686,500
Depreciation and Amortization	(154,800)	(84,500)	(380,900)	(342,100)
Other Operating Expenses	(1,146,800)	(1,140,800)	(3,230,100)	(3,535,700)
Operating Income/(Loss)	321,800	318,400	751,600	808,700
Interest Expense	(49,000)	(47,400)	(144,600)	(140,600)
Other Income/(Loss), Net	5,300	1,900	11,600	9,600
Net Income Attributable to Common Shareholders	170,100	167,500	381,300	418,900
Cash Flows Used for Investments in Plant			570,900	506,500
Assets	18,140,000		18,140,000		17,981,300
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	99,200	106,200	622,300	799,600
Depreciation and Amortization	(15,200)	(17,500)	(47,900)	(53,400)
Other Operating Expenses	(87,800)	(92,700)	(462,400)	(631,500)
Operating Income/(Loss)	(3,800)	(4,000)	112,000	114,700
Interest Expense	(10,200)	(9,200)	(30,800)	(27,200)
Other Income/(Loss), Net	600	(200)	500	(100)
Net Income Attributable to Common Shareholders	(7,000)	(3,700)	51,900	57,300
Cash Flows Used for Investments in Plant			170,300	120,000
Assets	3,149,900		3,149,900		3,104,500
Operating Segments [Member] | Electric Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	306,800	270,400	892,500	787,200
Depreciation and Amortization	(47,100)	(42,400)	(137,700)	(122,600)
Other Operating Expenses	(90,200)	(78,300)	(245,700)	(225,500)
Operating Income/(Loss)	169,500	149,700	509,100	439,100
Interest Expense	(26,900)	(25,900)	(82,200)	(79,800)
Other Income/(Loss), Net	6,300	3,800	14,200	11,900
Net Income Attributable to Common Shareholders	88,400	78,000	266,600	225,000
Cash Flows Used for Investments in Plant			536,200	493,900
Assets	8,454,400		8,454,400		8,019,300
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	211,500	211,600	636,800	655,200
Depreciation and Amortization	(8,600)	(7,100)	(23,100)	(21,500)
Other Operating Expenses	(179,300)	(199,600)	(564,700)	(619,400)
Operating Income/(Loss)	23,600	4,900	49,000	14,300
Interest Expense	(15,100)	(11,200)	(45,800)	(35,400)
Other Income/(Loss), Net	256,900	241,900	781,400	777,000
Net Income Attributable to Common Shareholders	268,500	$ 234,900	791,700	764,700
Cash Flows Used for Investments in Plant			81,800	$ 56,900
Assets	$ 13,624,800		$ 13,624,800		$ 13,256,700