SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,343,642	$ 2,175,797	$ 6,670,497	$ 6,476,084
Depreciation and Amortization	(302,000)	(296,500)	(851,900)	(840,500)
Impairment of Northern Pass Transmission	0	0	0	(239,644)
Other Operating Expenses	(1,480,400)	(1,370,100)	(4,284,600)	(4,241,100)
Operating Income/(Loss)	561,230	509,238	1,534,006	1,154,926
Interest Expense	(134,066)	(135,216)	(403,067)	(399,654)
Other Income, Net	29,218	26,968	83,565	103,818
Net Income/(Loss) Attributable to Common Shareholders	346,260	318,884	933,244	659,016
Cash Flows Used for Investments in Plant			2,101,564	2,129,037
Assets	43,507,345		43,507,345		$ 41,123,915
Other
Segment Reporting Information [Line Items]
Operating Revenues	322,300	258,100	900,700	767,100
Depreciation and Amortization	(24,100)	(17,200)	(68,100)	(45,600)
Impairment of Northern Pass Transmission				0
Other Operating Expenses	(271,500)	(275,900)	(771,400)	(719,800)
Operating Income/(Loss)	26,700	(35,000)	61,200	1,700
Interest Expense	(38,700)	(41,700)	(119,000)	(130,300)
Other Income, Net	387,000	497,000	1,161,700	813,700
Net Income/(Loss) Attributable to Common Shareholders	381,200	435,200	1,108,900	697,200
Cash Flows Used for Investments in Plant			181,200	173,600
Assets	22,230,100		22,230,100		20,469,600
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(602,600)	(529,300)	(1,784,300)	(1,599,500)
Depreciation and Amortization	300	600	1,200	1,700
Impairment of Northern Pass Transmission				0
Other Operating Expenses	603,100	638,400	1,787,900	1,708,200
Operating Income/(Loss)	800	109,700	4,800	110,400
Interest Expense	7,100	13,900	27,600	40,000
Other Income, Net	(383,000)	(545,100)	(1,149,900)	(833,600)
Net Income/(Loss) Attributable to Common Shareholders	(375,100)	(421,500)	(1,117,500)	(683,200)
Cash Flows Used for Investments in Plant			0	0
Assets	(20,969,100)		(20,969,100)		(19,488,800)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	2,060,300	1,931,600	5,481,900	5,327,200
Depreciation and Amortization	(182,000)	(188,400)	(490,000)	(517,600)
Impairment of Northern Pass Transmission				0
Other Operating Expenses	(1,577,000)	(1,448,200)	(4,309,900)	(4,166,800)
Operating Income/(Loss)	301,300	295,000	682,000	642,800
Interest Expense	(53,300)	(53,200)	(160,700)	(153,300)
Other Income, Net	16,000	13,900	44,900	44,200
Net Income/(Loss) Attributable to Common Shareholders	205,500	197,300	450,600	422,700
Cash Flows Used for Investments in Plant			781,200	861,000
Assets	23,864,700		23,864,700		22,541,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	110,200	103,400	770,000	779,900
Depreciation and Amortization	(14,800)	(16,100)	(55,100)	(56,000)
Impairment of Northern Pass Transmission				0
Other Operating Expenses	(104,200)	(99,300)	(592,000)	(620,400)
Operating Income/(Loss)	(8,800)	(12,000)	122,900	103,500
Interest Expense	(10,100)	(11,700)	(32,100)	(35,400)
Other Income, Net	700	0	2,500	1,100
Net Income/(Loss) Attributable to Common Shareholders	(14,000)	(17,100)	73,800	57,600
Cash Flows Used for Investments in Plant			322,800	327,800
Assets	4,579,900		4,579,900		4,345,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	389,300	347,800	1,135,600	1,037,900
Depreciation and Amortization	(70,600)	(64,300)	(206,800)	(188,000)
Impairment of Northern Pass Transmission				(239,600)
Other Operating Expenses	(120,600)	(159,600)	(338,500)	(366,900)
Operating Income/(Loss)	198,100	123,900	590,300	243,400
Interest Expense	(31,100)	(33,600)	(93,800)	(94,500)
Other Income, Net	6,900	61,400	22,700	78,200
Net Income/(Loss) Attributable to Common Shareholders	125,600	107,500	381,800	138,400
Cash Flows Used for Investments in Plant			732,400	686,700
Assets	11,469,700		11,469,700		10,904,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	64,100	64,200	166,600	163,500
Depreciation and Amortization	(10,800)	(11,100)	(33,100)	(35,000)
Impairment of Northern Pass Transmission				0
Other Operating Expenses	(10,200)	(25,500)	(60,700)	(75,400)
Operating Income/(Loss)	43,100	27,600	72,800	53,100
Interest Expense	(8,000)	(8,900)	(25,100)	(26,200)
Other Income, Net	1,600	(200)	1,700	200
Net Income/(Loss) Attributable to Common Shareholders	23,100	$ 17,500	35,600	26,300
Cash Flows Used for Investments in Plant			84,000	$ 79,900
Assets	$ 2,332,000		$ 2,332,000		$ 2,351,700