SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,050,400	$ 2,175,800	$ 1,884,500	$ 2,415,800	$ 2,034,900	$ 2,271,400	$ 1,853,900	$ 2,288,000	$ 8,526,470	$ 8,448,201	$ 7,751,952
Depreciation and Amortization									(1,080,700)	(1,072,000)	(863,800)
Impairment of Northern Pass Transmission			(239,600)						(239,644)	0	0
Other Operating Expenses									(5,615,700)	(5,676,300)	(5,000,000)
Operating Income	435,600	509,200	151,000	494,700	400,000	466,000	391,400	442,500	1,590,491	1,699,930	1,888,249
Interest Expense									(533,197)	(498,805)	(421,755)
Interest Income									12,800	18,100	8,300
Other Income/(Loss), Net									120,000	110,300	99,700
Income Tax (Expense)/Benefit									(273,499)	(288,972)	(578,892)
Net Income	251,900	320,800	33,300	310,600	233,200	291,300	244,600	271,400	916,572	1,040,519	995,515
Net Income Attributable to Noncontrolling Interests									(7,519)	(7,519)	(7,519)
Net Income/(Loss) Attributable to Common Shareholders	250,000	$ 318,900	$ 31,500	$ 308,700	231,300	$ 289,400	$ 242,800	$ 269,500	909,053	1,033,000	987,996
Total Assets (as of)	41,123,915				38,241,256				41,123,915	38,241,256
Cash Flows Used for Investments in Plant									2,911,489	2,569,936	2,348,105
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues									6,976,500	6,957,200	5,542,900
Depreciation and Amortization									(651,300)	(671,800)	(542,600)
Impairment of Northern Pass Transmission									0
Other Operating Expenses									(5,525,100)	(5,548,600)	(4,072,600)
Operating Income									800,100	736,800	927,700
Interest Expense									(206,400)	(202,800)	(186,300)
Interest Income									13,300	18,700	7,300
Other Income/(Loss), Net									46,800	67,500	41,600
Income Tax (Expense)/Benefit									(135,900)	(160,200)	(288,300)
Net Income									517,900	460,000	502,000
Net Income Attributable to Noncontrolling Interests									(4,600)	(4,600)	(4,600)
Net Income/(Loss) Attributable to Common Shareholders									513,300	455,400	497,400
Total Assets (as of)	22,541,900				21,389,100				22,541,900	21,389,100
Cash Flows Used for Investments in Plant									1,104,200	961,300	1,020,700
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues									1,062,200	1,022,200	947,300
Depreciation and Amortization									(68,300)	(75,000)	(72,900)
Impairment of Northern Pass Transmission									0
Other Operating Expenses									(830,800)	(787,600)	(716,400)
Operating Income									163,100	159,600	158,000
Interest Expense									(47,400)	(44,100)	(43,100)
Interest Income									100	0	100
Other Income/(Loss), Net									1,600	7,100	3,800
Income Tax (Expense)/Benefit									(21,200)	(29,400)	(44,200)
Net Income									96,200	93,200	74,600
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									96,200	93,200	74,600
Total Assets (as of)	4,345,500				3,904,900				4,345,500	3,904,900
Cash Flows Used for Investments in Plant									460,200	351,500	298,200
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues									1,389,000	1,286,300	1,301,700
Depreciation and Amortization									(253,300)	(231,800)	(209,400)
Impairment of Northern Pass Transmission									(239,600)
Other Operating Expenses									(411,200)	(375,500)	(382,800)
Operating Income									484,900	679,000	709,500
Interest Expense									(125,700)	(120,600)	(115,100)
Interest Income									1,500	2,400	1,800
Other Income/(Loss), Net									29,200	31,100	27,300
Income Tax (Expense)/Benefit									(130,500)	(161,800)	(228,700)
Net Income									259,400	430,100	394,800
Net Income Attributable to Noncontrolling Interests									(2,900)	(2,900)	(2,900)
Net Income/(Loss) Attributable to Common Shareholders									256,500	427,200	391,900
Total Assets (as of)	10,904,000				10,285,000				10,904,000	10,285,000
Cash Flows Used for Investments in Plant									987,000	976,200	867,600
Operating Segments | Water Distribution
Segment Reporting Information [Line Items]
Operating Revenues									214,600	212,000	15,900
Depreciation and Amortization									(46,900)	(46,500)	(3,700)
Impairment of Northern Pass Transmission									0
Other Operating Expenses									(101,000)	(99,800)	(8,300)
Operating Income									66,700	65,700	3,900
Interest Expense									(34,600)	(34,300)	(3,100)
Interest Income									0	0	100
Other Income/(Loss), Net									400	(400)	0
Income Tax (Expense)/Benefit									2,400	(100)	(2,100)
Net Income									34,900	30,900	(1,200)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									34,900	30,900	(1,200)
Total Assets (as of)	2,351,700				2,253,000				2,351,700	2,253,000
Cash Flows Used for Investments in Plant									118,000	102,300	16,000
Other
Segment Reporting Information [Line Items]
Operating Revenues									1,028,500	936,300	931,000
Depreciation and Amortization									(63,200)	(49,100)	(37,400)
Impairment of Northern Pass Transmission									0
Other Operating Expenses									(891,300)	(831,500)	(806,600)
Operating Income									74,000	55,700	87,000
Interest Expense									(170,300)	(129,300)	(90,000)
Interest Income									48,700	30,300	15,700
Other Income/(Loss), Net									1,041,500	1,185,300	1,113,000
Income Tax (Expense)/Benefit									11,700	62,500	(15,500)
Net Income									1,005,600	1,204,500	1,110,200
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									1,005,600	1,204,500	1,110,200
Total Assets (as of)	20,469,600				17,874,200				20,469,600	17,874,200
Cash Flows Used for Investments in Plant									242,100	178,600	145,600
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues									(2,144,300)	(1,965,800)	(986,800)
Depreciation and Amortization									2,300	2,200	2,200
Impairment of Northern Pass Transmission									0
Other Operating Expenses									2,143,700	1,966,700	986,700
Operating Income									1,700	3,100	2,100
Interest Expense									51,200	32,300	15,800
Interest Income									(50,800)	(33,300)	(16,700)
Other Income/(Loss), Net									(999,500)	(1,180,300)	(1,086,000)
Income Tax (Expense)/Benefit									0	0	(100)
Net Income									(997,400)	(1,178,200)	(1,084,900)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									(997,400)	(1,178,200)	(1,084,900)
Total Assets (as of)	$ (19,488,800)				$ (17,464,900)				(19,488,800)	(17,464,900)
Cash Flows Used for Investments in Plant									$ 0	$ 0	$ 0