SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,175,797	$ 2,271,425	$ 6,476,084	$ 6,413,243
Depreciation and Amortization	(296,500)	(301,400)	(840,500)	(786,200)
Impairment of Northern Pass Transmission	0	0	(239,644)	0
Other Operating Expenses	(1,370,100)	(1,504,000)	(4,241,100)	(4,327,100)
Operating Income/(Loss)	509,238	466,021	1,154,926	1,299,907
Interest Expense	(135,216)	(125,201)	(399,654)	(372,734)
Other Income/(Loss), Net	26,968	16,718	103,818	100,656
Net Income/(Loss) Attributable to Common Shareholders	318,884	289,380	659,016	801,693
Cash Flows Used for Investments in Plant			2,129,037	1,885,081
Assets	39,725,394		39,725,394		$ 38,241,256
Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues
Other
Segment Reporting Information [Line Items]
Operating Revenues	258,100	224,600	767,100	692,500
Depreciation and Amortization	(17,200)	(12,000)	(45,600)	(35,900)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(275,900)	(200,900)	(719,800)	(617,200)
Operating Income/(Loss)	(35,000)	11,700	1,700	39,400
Interest Expense	(41,700)	(30,500)	(130,300)	(94,800)
Other Income/(Loss), Net	497,000	251,700	813,700	913,800
Net Income/(Loss) Attributable to Common Shareholders	435,200	271,100	697,200	905,900
Cash Flows Used for Investments in Plant			173,600	118,300
Assets	19,137,800		19,137,800		17,874,200
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(529,300)	(483,700)	(1,599,500)	(1,456,100)
Depreciation and Amortization	600	600	1,700	1,700
Impairment of Northern Pass Transmission			0
Other Operating Expenses	638,400	483,700	1,708,200	1,457,000
Operating Income/(Loss)	109,700	600	110,400	2,600
Interest Expense	13,900	7,800	40,000	23,200
Other Income/(Loss), Net	(545,100)	(278,400)	(833,600)	(915,400)
Net Income/(Loss) Attributable to Common Shareholders	(421,500)	(270,000)	(683,200)	(889,600)
Cash Flows Used for Investments in Plant			0	0
Assets	(18,494,100)		(18,494,100)		(17,464,900)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,931,600	2,027,400	5,327,200	5,321,800
Depreciation and Amortization	(188,400)	(206,100)	(517,600)	(486,000)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(1,448,200)	(1,562,200)	(4,166,800)	(4,238,600)
Operating Income/(Loss)	295,000	259,100	642,800	597,200
Interest Expense	(53,200)	(52,400)	(153,300)	(152,000)
Other Income/(Loss), Net	13,900	32,200	44,200	70,900
Net Income/(Loss) Attributable to Common Shareholders	197,300	173,800	422,700	379,300
Cash Flows Used for Investments in Plant			861,000	717,400
Assets	21,999,400		21,999,400		21,389,100
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	103,400	109,000	779,900	740,500
Depreciation and Amortization	(16,100)	(13,900)	(56,000)	(59,600)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(99,300)	(102,400)	(620,400)	(585,700)
Operating Income/(Loss)	(12,000)	(7,300)	103,500	95,200
Interest Expense	(11,700)	(11,300)	(35,400)	(33,700)
Other Income/(Loss), Net	0	1,500	1,100	5,100
Net Income/(Loss) Attributable to Common Shareholders	(17,100)	(12,600)	57,600	50,200
Cash Flows Used for Investments in Plant			327,800	245,500
Assets	4,103,500		4,103,500		3,904,900
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	347,800	330,600	1,037,900	953,000
Depreciation and Amortization	(64,300)	(58,300)	(188,000)	(171,800)
Impairment of Northern Pass Transmission			(239,600)
Other Operating Expenses	(159,600)	(96,700)	(366,900)	(268,700)
Operating Income/(Loss)	123,900	175,600	243,400	512,500
Interest Expense	(33,600)	(30,300)	(94,500)	(89,900)
Other Income/(Loss), Net	61,400	9,000	78,200	26,700
Net Income/(Loss) Attributable to Common Shareholders	107,500	109,500	138,400	329,600
Cash Flows Used for Investments in Plant			686,700	735,800
Assets	10,666,900		10,666,900		10,285,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	64,200	63,500	163,500	161,500
Depreciation and Amortization	(11,100)	(11,700)	(35,000)	(34,600)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(25,500)	(25,500)	(75,400)	(73,900)
Operating Income/(Loss)	27,600	26,300	53,100	53,000
Interest Expense	(8,900)	(8,500)	(26,200)	(25,500)
Other Income/(Loss), Net	(200)	700	200	(400)
Net Income/(Loss) Attributable to Common Shareholders	17,500	$ 17,600	26,300	26,300
Cash Flows Used for Investments in Plant			79,900	$ 68,100
Assets	$ 2,311,900		$ 2,311,900		$ 2,253,000