SEGMENT INFORMATION - Summary of Segment Information and Segmented Total Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,884,495	$ 1,853,856	$ 4,300,287	$ 4,141,818
Depreciation and Amortization	(258,000)	(235,300)	(544,000)	(484,800)
Impairment of Northern Pass Transmission	(239,644)	0	(239,644)	0
Other Operating Expenses	(1,235,900)	(1,227,200)	(2,871,000)	(2,823,100)
Operating Income/(Loss)	150,988	391,354	645,688	833,886
Interest Expense	(132,705)	(126,404)	(264,438)	(247,533)
Other Income/(Loss), Net	45,866	50,149	76,850	83,938
Net Income Attributable to Common Shareholders	31,454	242,767	340,132	512,313
Cash Flows Used for Investments in Plant			1,377,753	1,251,678
Assets	38,995,906		38,995,906		$ 38,241,256
Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues
Other
Segment Reporting Information [Line Items]
Operating Revenues	250,800	237,000	509,000	467,900
Depreciation and Amortization	(14,700)	(11,900)	(28,300)	(23,900)
Impairment of Northern Pass Transmission	0		0
Other Operating Expenses	(218,400)	(211,700)	(444,000)	(416,300)
Operating Income/(Loss)	17,700	13,400	36,700	27,700
Interest Expense	(44,500)	(32,300)	(88,600)	(64,300)
Other Income/(Loss), Net	(114,900)	302,000	316,800	662,100
Net Income Attributable to Common Shareholders	(145,600)	288,800	262,000	634,800
Cash Flows Used for Investments in Plant			103,200	77,100
Assets	18,550,400		18,550,400		17,874,200
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(539,100)	(500,400)	(1,070,200)	(972,400)
Depreciation and Amortization	600	600	1,100	1,100
Impairment of Northern Pass Transmission	0		0
Other Operating Expenses	538,700	501,400	1,069,800	973,300
Operating Income/(Loss)	200	1,600	700	2,000
Interest Expense	13,600	8,000	26,100	15,400
Other Income/(Loss), Net	139,100	(281,800)	(288,500)	(637,000)
Net Income Attributable to Common Shareholders	152,900	(272,200)	(261,700)	(619,600)
Cash Flows Used for Investments in Plant			0	0
Assets	(18,004,800)		(18,004,800)		(17,464,900)
Electric Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	1,561,800	1,539,900	3,395,600	3,294,500
Depreciation and Amortization	(150,100)	(135,500)	(329,300)	(279,900)
Impairment of Northern Pass Transmission	0		0
Other Operating Expenses	(1,243,000)	(1,233,000)	(2,718,600)	(2,676,500)
Operating Income/(Loss)	168,700	171,400	347,700	338,100
Interest Expense	(50,800)	(52,100)	(100,000)	(99,500)
Other Income/(Loss), Net	12,200	19,000	30,400	38,600
Net Income Attributable to Common Shareholders	105,400	101,300	225,400	205,500
Cash Flows Used for Investments in Plant			571,300	475,600
Assets	21,628,800		21,628,800		21,389,100
Natural Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	207,700	206,500	676,600	631,400
Depreciation and Amortization	(19,500)	(19,300)	(39,900)	(45,700)
Impairment of Northern Pass Transmission	0		0
Other Operating Expenses	(179,800)	(170,600)	(521,100)	(483,200)
Operating Income/(Loss)	8,400	16,600	115,600	102,500
Interest Expense	(11,900)	(11,400)	(23,700)	(22,500)
Other Income/(Loss), Net	700	1,600	1,000	3,500
Net Income Attributable to Common Shareholders	(1,800)	5,000	74,700	62,800
Cash Flows Used for Investments in Plant			202,700	150,300
Assets	4,031,300		4,031,300		3,904,900
Electric Transmission | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	349,600	317,400	690,100	622,400
Depreciation and Amortization	(62,300)	(57,000)	(123,700)	(113,500)
Impairment of Northern Pass Transmission	(239,600)		(239,600)
Other Operating Expenses	(108,400)	(88,800)	(207,200)	(172,000)
Operating Income/(Loss)	(60,700)	171,600	119,600	336,900
Interest Expense	(30,500)	(30,000)	(61,000)	(59,700)
Other Income/(Loss), Net	8,700	9,900	16,800	17,800
Net Income Attributable to Common Shareholders	(87,400)	112,700	30,900	220,100
Cash Flows Used for Investments in Plant			449,200	508,500
Assets	10,499,700		10,499,700		10,285,000
Water Distribution | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues	53,700	53,500	99,200	98,000
Depreciation and Amortization	(12,000)	(12,200)	(23,900)	(22,900)
Impairment of Northern Pass Transmission	0		0
Other Operating Expenses	(25,000)	(24,500)	(49,900)	(48,400)
Operating Income/(Loss)	16,700	16,800	25,400	26,700
Interest Expense	(8,600)	(8,600)	(17,200)	(16,900)
Other Income/(Loss), Net	100	(600)	400	(1,100)
Net Income Attributable to Common Shareholders	8,000	$ 7,200	8,800	8,700
Cash Flows Used for Investments in Plant			51,400	$ 40,200
Assets	$ 2,290,500		$ 2,290,500		$ 2,253,000