SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,691,200	$ 1,933,100	$ 1,817,100	$ 2,513,400	$ 1,881,200	$ 1,892,500	$ 1,677,600	$ 2,290,600	$ 7,954,827	$ 7,741,856	$ 7,301,204
Operating Income	385,500	469,200	412,000	497,500	430,200	440,900	294,000	467,700	1,764,164	1,632,849	1,529,435
Total Interest Expense									(372,420)	(362,106)	(338,699)
Other Income/(Loss), Net									34,227	24,619	29,894
Income Tax Expense									(539,967)	(468,297)	(426,941)
Net Income	183,700	237,800	209,400	255,100	223,600	236,500	129,200	237,800	886,004	827,065	793,689
Net Income Attributable to Noncontrolling Interests									(7,519)	(7,519)	(7,682)
Net Income Attributable to Controlling Interests	181,800	$ 235,900	$ 207,500	$ 253,300	221,500	$ 234,600	$ 127,400	$ 236,000	878,485	819,546	786,007
Total Assets	30,580,309				29,740,387				30,580,309	29,740,387
Investments in Property, Plant and Equipment									1,724,139	1,603,744	1,456,787
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues									5,903,600	5,663,400	5,362,300
Depreciation And Amortization									(425,200)	(384,600)	(604,800)
Operating Expenses, Other									(4,470,200)	(4,366,200)	(3,927,700)
Operating Income									1,008,200	912,600	829,800
Total Interest Expense									(186,300)	(191,600)	(175,000)
Interest Income									5,700	5,100	4,100
Other Income/(Loss) Excluding Interest Income									7,200	10,700	12,900
Income Tax Expense									(322,800)	(269,700)	(240,000)
Net Income									512,000	467,100	431,800
Net Income Attributable to Noncontrolling Interests									(4,700)	(4,700)	(4,800)
Net Income Attributable to Controlling Interests									507,300	462,400	427,000
Total Assets	17,981,300				17,536,900				17,981,300	17,536,900
Investments in Property, Plant and Equipment									718,900	645,200	639,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues									995,500	1,007,300	855,800
Depreciation And Amortization									(70,500)	(68,100)	(66,700)
Operating Expenses, Other									(776,700)	(786,700)	(659,400)
Operating Income									148,300	152,500	129,700
Total Interest Expense									(36,900)	(34,000)	(33,100)
Interest Income									100	0	0
Other Income/(Loss) Excluding Interest Income									800	200	800
Income Tax Expense									(40,100)	(46,400)	(36,500)
Net Income									72,200	72,300	60,900
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									72,200	72,300	60,900
Total Assets	3,104,500				3,029,300				3,104,500	3,029,300
Investments in Property, Plant and Equipment									182,200	176,700	168,100
Electric Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues									1,069,100	1,018,200	978,700
Depreciation And Amortization									(165,600)	(150,500)	(136,200)
Operating Expenses, Other									(314,900)	(302,100)	(281,800)
Operating Income									588,600	565,600	560,700
Total Interest Expense									(105,800)	(104,100)	(100,300)
Interest Income									1,600	900	700
Other Income/(Loss) Excluding Interest Income									14,500	10,300	10,900
Income Tax Expense									(191,600)	(174,500)	(182,100)
Net Income									307,300	298,200	289,900
Net Income Attributable to Noncontrolling Interests									(2,800)	(2,800)	(2,900)
Net Income Attributable to Controlling Interests									304,500	295,400	287,000
Total Assets	8,019,300				7,615,600				8,019,300	7,615,600
Investments in Property, Plant and Equipment									749,100	731,600	618,500
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									863,600	790,900	777,500
Depreciation And Amortization									(29,000)	(42,100)	(62,200)
Operating Expenses, Other									(817,900)	(748,000)	(715,000)
Operating Income									16,700	800	300
Total Interest Expense									(48,000)	(36,600)	(35,500)
Interest Income									4,400	3,600	5,400
Other Income/(Loss) Excluding Interest Income									977,800	916,000	858,900
Income Tax Expense									14,500	22,300	31,900
Net Income									965,400	906,100	861,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									965,400	906,100	861,000
Total Assets	13,256,700				12,664,900				13,256,700	12,664,900
Investments in Property, Plant and Equipment									73,900	50,200	31,200
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									(877,000)	(737,900)	(673,100)
Depreciation And Amortization									2,100	19,900	10,200
Operating Expenses, Other									877,300	719,300	671,800
Operating Income									2,400	1,300	8,900
Total Interest Expense									4,600	4,200	5,200
Interest Income									(5,100)	(3,600)	(5,600)
Other Income/(Loss) Excluding Interest Income									(972,800)	(918,600)	(858,200)
Income Tax Expense									0	0	(200)
Net Income									(970,900)	(916,700)	(849,900)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									(970,900)	(916,700)	(849,900)
Total Assets	(11,781,500)				(11,106,300)				(11,781,500)	(11,106,300)
Investments in Property, Plant and Equipment									0	0	0
Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues									7,954,800	7,741,900	7,301,200
Depreciation And Amortization									(688,200)	(625,400)	(859,700)
Operating Expenses, Other									(5,502,400)	(5,483,700)	(4,912,100)
Operating Income									1,764,200	1,632,800	1,529,400
Total Interest Expense									(372,400)	(362,100)	(338,700)
Interest Income									6,700	6,000	4,600
Other Income/(Loss) Excluding Interest Income									27,500	18,600	25,300
Income Tax Expense									(540,000)	(468,300)	(426,900)
Net Income									886,000	827,000	793,700
Net Income Attributable to Noncontrolling Interests									(7,500)	(7,500)	(7,700)
Net Income Attributable to Controlling Interests									878,500	819,500	786,000
Total Assets	$ 30,580,300				$ 29,740,400				30,580,300	29,740,400
Investments in Property, Plant and Equipment									$ 1,724,100	$ 1,603,700	$ 1,456,800