SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,953,128	$ 1,884,495	$ 4,326,854	$ 4,300,287
Depreciation and Amortization	(263,900)	(258,000)	(549,900)	(544,000)
Impairment of Northern Pass Transmission	0	(239,644)	0	(239,644)
Other Operating Expenses	(1,255,600)	(1,235,900)	(2,804,200)	(2,871,000)
Operating Income	433,655	150,988	972,775	645,688
Interest Expense	(134,285)	(132,705)	(269,000)	(264,438)
Other Income, Net	30,243	45,866	54,347	76,850
Net Income/(Loss) Attributable to Common Shareholders	252,232	31,454	586,984	340,132
Cash Flows Used for Investments in Plant			1,400,198	1,377,753
Assets	42,038,718		42,038,718		$ 41,123,915
Other
Segment Reporting Information [Line Items]
Operating Revenues	283,800	250,800	578,500	509,000
Depreciation and Amortization	(22,700)	(14,700)	(44,000)	(28,300)
Impairment of Northern Pass Transmission		0		0
Other Operating Expenses	(243,100)	(218,400)	(500,000)	(444,000)
Operating Income	18,000	17,700	34,500	36,700
Interest Expense	(37,600)	(44,500)	(80,300)	(88,600)
Other Income, Net	306,500	(114,900)	774,700	316,800
Net Income/(Loss) Attributable to Common Shareholders	285,400	(145,600)	727,800	262,000
Cash Flows Used for Investments in Plant			124,600	103,200
Assets	21,202,000		21,202,000		20,469,600
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(590,300)	(539,100)	(1,181,700)	(1,070,200)
Depreciation and Amortization	300	600	900	1,100
Impairment of Northern Pass Transmission		0		0
Other Operating Expenses	593,500	538,700	1,184,800	1,069,800
Operating Income	3,500	200	4,000	700
Interest Expense	8,800	13,600	20,500	26,100
Other Income, Net	(303,700)	139,100	(766,900)	(288,500)
Net Income/(Loss) Attributable to Common Shareholders	(291,400)	152,900	(742,400)	(261,700)
Cash Flows Used for Investments in Plant			0	0
Assets	(20,452,000)		(20,452,000)		(19,488,800)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,616,200	1,561,800	3,421,600	3,395,600
Depreciation and Amortization	(140,700)	(150,100)	(308,100)	(329,300)
Impairment of Northern Pass Transmission		0		0
Other Operating Expenses	(1,293,300)	(1,243,000)	(2,732,900)	(2,718,600)
Operating Income	182,200	168,700	380,600	347,700
Interest Expense	(54,300)	(50,800)	(107,400)	(100,000)
Other Income, Net	16,400	12,200	28,900	30,400
Net Income/(Loss) Attributable to Common Shareholders	115,000	105,400	245,100	225,400
Cash Flows Used for Investments in Plant			563,200	571,300
Assets	23,166,100		23,166,100		22,541,900
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	211,200	207,700	659,800	676,600
Depreciation and Amortization	(21,200)	(19,500)	(40,300)	(39,900)
Impairment of Northern Pass Transmission		0		0
Other Operating Expenses	(176,100)	(179,800)	(487,700)	(521,100)
Operating Income	13,900	8,400	131,800	115,600
Interest Expense	(10,700)	(11,900)	(22,000)	(23,700)
Other Income, Net	400	700	1,800	1,000
Net Income/(Loss) Attributable to Common Shareholders	3,300	(1,800)	87,800	74,700
Cash Flows Used for Investments in Plant			205,600	202,700
Assets	4,503,300		4,503,300		4,345,500
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	376,700	349,600	746,300	690,100
Depreciation and Amortization	(68,800)	(62,300)	(136,200)	(123,700)
Impairment of Northern Pass Transmission		(239,600)		(239,600)
Other Operating Expenses	(111,200)	(108,400)	(218,000)	(207,200)
Operating Income	196,700	(60,700)	392,100	119,600
Interest Expense	(32,100)	(30,500)	(62,700)	(61,000)
Other Income, Net	10,600	8,700	15,700	16,800
Net Income/(Loss) Attributable to Common Shareholders	129,500	(87,400)	256,200	30,900
Cash Flows Used for Investments in Plant			460,800	449,200
Assets	11,224,700		11,224,700		10,904,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	55,500	53,700	102,400	99,200
Depreciation and Amortization	(10,800)	(12,000)	(22,200)	(23,900)
Impairment of Northern Pass Transmission		0		0
Other Operating Expenses	(25,400)	(25,000)	(50,400)	(49,900)
Operating Income	19,300	16,700	29,800	25,400
Interest Expense	(8,400)	(8,600)	(17,100)	(17,200)
Other Income, Net	0	100	100	400
Net Income/(Loss) Attributable to Common Shareholders	10,400	$ 8,000	12,500	8,800
Cash Flows Used for Investments in Plant			46,000	$ 51,400
Assets	$ 2,394,600		$ 2,394,600		$ 2,351,700