SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,895,600	$ 1,988,500	$ 1,762,800	$ 2,105,100	$ 1,776,600	$ 2,039,700	$ 1,767,200	$ 2,055,600	$ 7,751,952	$ 7,639,129	$ 7,954,827
Depreciation and Amortization									(863,800)	(787,100)	(688,200)
Other Operating Expenses									(4,970,000)	(4,992,100)	(5,502,400)
Operating Income	450,900	502,600	455,700	509,000	438,100	509,900	423,400	488,500	1,918,154	1,859,859	1,764,164
Interest Expense									(421,755)	(400,961)	(372,420)
Interest Income									8,300	11,000	6,700
Other Income, Net									69,700	34,900	27,500
Income Tax Expense									(578,892)	(554,997)	(539,967)
Net Income	239,400	262,200	232,600	261,300	231,100	267,200	205,500	246,000	995,515	949,821	886,004
Net Income Attributable to Noncontrolling Interests									(7,519)	(7,519)	(7,519)
Net Income Attributable to Common Shareholders	237,400	$ 260,400	$ 230,700	$ 259,500	229,200	$ 265,300	$ 203,600	$ 244,200	987,996	942,302	878,485
Total Assets (as of)	36,220,386				32,053,173				36,220,386	32,053,173
Cash Flows Used for Investments in Plant									2,348,105	1,976,867	1,724,139
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues									5,542,900	5,594,300	5,903,600
Depreciation and Amortization									(542,600)	(504,700)	(425,200)
Other Operating Expenses									(4,046,000)	(4,155,100)	(4,470,200)
Operating Income									954,300	934,500	1,008,200
Interest Expense									(186,300)	(193,100)	(186,300)
Interest Income									7,300	10,000	5,700
Other Income, Net									15,000	4,800	7,200
Income Tax Expense									(288,300)	(288,800)	(322,800)
Net Income									502,000	467,400	512,000
Net Income Attributable to Noncontrolling Interests									(4,600)	(4,600)	(4,700)
Net Income Attributable to Common Shareholders									497,400	462,800	507,300
Total Assets (as of)	19,250,400				18,367,500				19,250,400	18,367,500
Cash Flows Used for Investments in Plant									1,020,700	812,600	718,900
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues									947,300	857,700	995,500
Depreciation and Amortization									(72,900)	(65,300)	(70,500)
Other Operating Expenses									(713,500)	(628,900)	(776,700)
Operating Income									160,900	163,500	148,300
Interest Expense									(43,100)	(41,300)	(36,900)
Interest Income									100	100	100
Other Income, Net									900	600	800
Income Tax Expense									(44,200)	(45,200)	(40,100)
Net Income									74,600	77,700	72,200
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Common Shareholders									74,600	77,700	72,200
Total Assets (as of)	3,595,200				3,303,800				3,595,200	3,303,800
Cash Flows Used for Investments in Plant									298,200	255,300	182,200
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues									1,301,700	1,210,000	1,069,100
Depreciation and Amortization									(209,400)	(185,800)	(165,600)
Other Operating Expenses									(382,600)	(321,800)	(314,900)
Operating Income									709,700	702,400	588,600
Interest Expense									(115,100)	(110,000)	(105,800)
Interest Income									1,800	1,200	1,600
Other Income, Net									27,100	18,300	14,500
Income Tax Expense									(228,700)	(238,200)	(191,600)
Net Income									394,800	373,700	307,300
Net Income Attributable to Noncontrolling Interests									(2,900)	(2,900)	(2,800)
Net Income Attributable to Common Shareholders									391,900	370,800	304,500
Total Assets (as of)	9,401,200				8,751,500				9,401,200	8,751,500
Cash Flows Used for Investments in Plant									867,600	801,000	749,100
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating Revenues									946,900	870,400	863,600
Depreciation and Amortization									(41,100)	(33,500)	(29,000)
Other Operating Expenses									(814,600)	(778,100)	(817,900)
Operating Income									91,200	58,800	16,700
Interest Expense									(93,100)	(63,500)	(48,000)
Interest Income									15,800	7,000	4,400
Other Income, Net									1,112,700	1,020,100	977,800
Income Tax Expense									(17,600)	16,500	14,500
Net Income									1,109,000	1,038,900	965,400
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Common Shareholders									1,109,000	1,038,900	965,400
Total Assets (as of)	18,403,800				14,493,100				18,403,800	14,493,100
Cash Flows Used for Investments in Plant									161,600	108,000	73,900
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues									(986,800)	(893,300)	(877,000)
Depreciation and Amortization									2,200	2,200	2,100
Other Operating Expenses									986,700	891,800	877,300
Operating Income									2,100	700	2,400
Interest Expense									15,800	6,900	4,600
Interest Income									(16,700)	(7,300)	(5,100)
Other Income, Net									(1,086,000)	(1,008,900)	(972,800)
Income Tax Expense									(100)	700	0
Net Income									(1,084,900)	(1,007,900)	(970,900)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Common Shareholders									(1,084,900)	(1,007,900)	(970,900)
Total Assets (as of)	$ (14,430,200)				$ (12,862,700)				(14,430,200)	(12,862,700)
Cash Flows Used for Investments in Plant									$ 0	$ 0	$ 0