SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,776,600	$ 2,039,700	$ 1,767,200	$ 2,055,600	$ 1,691,200	$ 1,933,100	$ 1,817,100	$ 2,513,400	$ 7,639,129	$ 7,954,827	$ 7,741,856
Depreciation and Amortization									(787,100)	(688,200)	(625,400)
Other Operating Expenses									(4,992,100)	(5,502,400)	(5,483,700)
Operating Income	438,100	509,900	423,400	488,500	385,500	469,200	412,000	497,500	1,859,859	1,764,164	1,632,849
Interest Expense									(400,961)	(372,420)	(362,106)
Interest Income									11,000	6,700	6,000
Other Income, Net									34,900	27,500	18,600
Tax Effect									(554,997)	(539,967)	(468,297)
Net Income	231,100	267,200	205,500	246,000	183,700	237,800	209,400	255,100	949,821	886,004	827,065
Net Income (Loss) Attributable to Noncontrolling Interest									(7,519)	(7,519)	(7,519)
Net Income Attributable to Common Shareholders	229,200	$ 265,300	$ 203,600	$ 244,200	181,800	$ 235,900	$ 207,500	$ 253,300	942,302	878,485	819,546
Total Assets (as of)	32,053,173				30,580,309				32,053,173	30,580,309
Cash Flows Used for Investments in Plant									1,976,867	1,724,139	1,603,744
Operating Segments [Member] | Electric Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues									5,594,300	5,903,600	5,663,400
Depreciation and Amortization									(504,700)	(425,200)	(384,600)
Other Operating Expenses									(4,155,100)	(4,470,200)	(4,366,200)
Operating Income									934,500	1,008,200	912,600
Interest Expense									(193,100)	(186,300)	(191,600)
Interest Income									10,000	5,700	5,100
Other Income, Net									4,800	7,200	10,700
Tax Effect									(288,800)	(322,800)	(269,700)
Net Income									467,400	512,000	467,100
Net Income (Loss) Attributable to Noncontrolling Interest									(4,600)	(4,700)	(4,700)
Net Income Attributable to Common Shareholders									462,800	507,300	462,400
Total Assets (as of)	18,367,500				17,981,300				18,367,500	17,981,300
Cash Flows Used for Investments in Plant									812,600	718,900	645,200
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues									857,700	995,500	1,007,300
Depreciation and Amortization									(65,300)	(70,500)	(68,100)
Other Operating Expenses									(628,900)	(776,700)	(786,700)
Operating Income									163,500	148,300	152,500
Interest Expense									(41,300)	(36,900)	(34,000)
Interest Income									100	100	0
Other Income, Net									600	800	200
Tax Effect									(45,200)	(40,100)	(46,400)
Net Income									77,700	72,200	72,300
Net Income (Loss) Attributable to Noncontrolling Interest									0	0	0
Net Income Attributable to Common Shareholders									77,700	72,200	72,300
Total Assets (as of)	3,303,800				3,104,500				3,303,800	3,104,500
Cash Flows Used for Investments in Plant									255,300	182,200	176,700
Operating Segments [Member] | Electric Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues									1,210,000	1,069,100	1,018,200
Depreciation and Amortization									(185,800)	(165,600)	(150,500)
Other Operating Expenses									(321,800)	(314,900)	(302,100)
Operating Income									702,400	588,600	565,600
Interest Expense									(110,000)	(105,800)	(104,100)
Interest Income									1,200	1,600	900
Other Income, Net									18,300	14,500	10,300
Tax Effect									(238,200)	(191,600)	(174,500)
Net Income									373,700	307,300	298,200
Net Income (Loss) Attributable to Noncontrolling Interest									(2,900)	(2,800)	(2,800)
Net Income Attributable to Common Shareholders									370,800	304,500	295,400
Total Assets (as of)	8,751,500				8,019,300				8,751,500	8,019,300
Cash Flows Used for Investments in Plant									801,000	749,100	731,600
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues									870,400	863,600	790,900
Depreciation and Amortization									(33,500)	(29,000)	(42,100)
Other Operating Expenses									(778,100)	(817,900)	(748,000)
Operating Income									58,800	16,700	800
Interest Expense									(63,500)	(48,000)	(36,600)
Interest Income									7,000	4,400	3,600
Other Income, Net									1,020,100	977,800	916,000
Tax Effect									16,500	14,500	22,300
Net Income									1,038,900	965,400	906,100
Net Income (Loss) Attributable to Noncontrolling Interest									0	0	0
Net Income Attributable to Common Shareholders									1,038,900	965,400	906,100
Total Assets (as of)	14,493,100				13,256,700				14,493,100	13,256,700
Cash Flows Used for Investments in Plant									108,000	73,900	50,200
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Revenues									(893,300)	(877,000)	(737,900)
Depreciation and Amortization									2,200	2,100	19,900
Other Operating Expenses									891,800	877,300	719,300
Operating Income									700	2,400	1,300
Interest Expense									6,900	4,600	4,200
Interest Income									(7,300)	(5,100)	(3,600)
Other Income, Net									(1,008,900)	(972,800)	(918,600)
Tax Effect									700	0	0
Net Income									(1,007,900)	(970,900)	(916,700)
Net Income (Loss) Attributable to Noncontrolling Interest									0	0	0
Net Income Attributable to Common Shareholders									(1,007,900)	(970,900)	(916,700)
Total Assets (as of)	$ (12,862,700)				$ (11,781,500)				(12,862,700)	(11,781,500)
Cash Flows Used for Investments in Plant									$ 0	$ 0	$ 0