SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating Revenues	$ 8,904,430	$ 8,526,470	$ 8,448,201
Depreciation and Amortization	(1,159,100)	(1,080,700)	(1,072,000)
Impairment of Northern Pass Transmission	0	(239,644)	0
Other Operating Expenses	(5,756,600)	(5,615,700)	(5,676,300)
Operating Income	1,988,734	1,590,491	1,699,930
Interest Expense	(538,452)	(533,197)	(498,805)
Interest Income	4,800	12,800	18,100
Other Income, Net	103,800	120,000	110,300
Income Tax (Expense)/Benefit	(346,186)	(273,499)	(288,972)
Net Income	1,212,686	916,572	1,040,519
Net Income Attributable to Noncontrolling Interests	(7,519)	(7,519)	(7,519)
Net Income Attributable to Common Shareholders	1,205,167	909,053	1,033,000
Total Assets (as of)	46,099,598	41,123,915
Cash Flows Used for Investments in Plant	2,942,996	2,911,489	2,569,936
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues	7,132,300	6,976,500	6,957,200
Depreciation and Amortization	(657,000)	(651,300)	(671,800)
Impairment of Northern Pass Transmission		0
Other Operating Expenses	(5,642,300)	(5,525,100)	(5,548,600)
Operating Income	833,000	800,100	736,800
Interest Expense	(216,000)	(206,400)	(202,800)
Interest Income	3,200	13,300	18,700
Other Income, Net	58,000	46,800	67,500
Income Tax (Expense)/Benefit	(129,600)	(135,900)	(160,200)
Net Income	548,600	517,900	460,000
Net Income Attributable to Noncontrolling Interests	(4,600)	(4,600)	(4,600)
Net Income Attributable to Common Shareholders	544,000	513,300	455,400
Total Assets (as of)	24,981,900	22,541,900
Cash Flows Used for Investments in Plant	1,079,000	1,104,200	961,300
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues	1,208,700	1,062,200	1,022,200
Depreciation and Amortization	(87,900)	(68,300)	(75,000)
Impairment of Northern Pass Transmission		0
Other Operating Expenses	(913,800)	(830,800)	(787,600)
Operating Income	207,000	163,100	159,600
Interest Expense	(40,000)	(47,400)	(44,100)
Interest Income	900	100	0
Other Income, Net	3,100	1,600	7,100
Income Tax (Expense)/Benefit	(36,900)	(21,200)	(29,400)
Net Income	134,100	96,200	93,200
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	134,100	96,200	93,200
Total Assets (as of)	6,450,500	4,345,500
Cash Flows Used for Investments in Plant	494,400	460,200	351,500
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues	1,536,100	1,389,000	1,286,300
Depreciation and Amortization	(278,100)	(253,300)	(231,800)
Impairment of Northern Pass Transmission		(239,600)
Other Operating Expenses	(470,000)	(411,200)	(375,500)
Operating Income	788,000	484,900	679,000
Interest Expense	(126,800)	(125,700)	(120,600)
Interest Income	4,700	1,500	2,400
Other Income, Net	23,300	29,200	31,100
Income Tax (Expense)/Benefit	(183,800)	(130,500)	(161,800)
Net Income	505,400	259,400	430,100
Net Income Attributable to Noncontrolling Interests	(2,900)	(2,900)	(2,900)
Net Income Attributable to Common Shareholders	502,500	256,500	427,200
Total Assets (as of)	11,695,000	10,904,000
Cash Flows Used for Investments in Plant	1,004,600	987,000	976,200
Operating Segments | Water Distribution
Segment Reporting Information [Line Items]
Operating Revenues	215,400	214,600	212,000
Depreciation and Amortization	(44,200)	(46,900)	(46,500)
Impairment of Northern Pass Transmission		0
Other Operating Expenses	(86,600)	(101,000)	(99,800)
Operating Income	84,600	66,700	65,700
Interest Expense	(32,900)	(34,600)	(34,300)
Interest Income	0	0	0
Other Income, Net	2,000	400	(400)
Income Tax (Expense)/Benefit	(12,500)	2,400	(100)
Net Income	41,200	34,900	30,900
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	41,200	34,900	30,900
Total Assets (as of)	2,375,200	2,351,700
Cash Flows Used for Investments in Plant	118,800	118,000	102,300
Other
Segment Reporting Information [Line Items]
Operating Revenues	1,235,900	1,028,500	936,300
Depreciation and Amortization	(93,500)	(63,200)	(49,100)
Impairment of Northern Pass Transmission		0
Other Operating Expenses	(1,071,900)	(891,300)	(831,500)
Operating Income	70,500	74,000	55,700
Interest Expense	(161,000)	(170,300)	(129,300)
Interest Income	37,800	48,700	30,300
Other Income, Net	1,382,900	945,300	1,092,100
Income Tax (Expense)/Benefit	16,600	11,700	62,500
Net Income	1,346,800	909,400	1,111,300
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	1,346,800	909,400	1,111,300
Total Assets (as of)	22,089,400	18,843,700
Cash Flows Used for Investments in Plant	246,200	242,100	178,600
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(2,424,000)	(2,144,300)	(1,965,800)
Depreciation and Amortization	1,600	2,300	2,200
Impairment of Northern Pass Transmission		0
Other Operating Expenses	2,428,000	2,143,700	1,966,700
Operating Income	5,600	1,700	3,100
Interest Expense	38,300	51,200	32,300
Interest Income	(41,800)	(50,800)	(33,300)
Other Income, Net	(1,365,500)	(903,300)	(1,087,100)
Income Tax (Expense)/Benefit	0	0	0
Net Income	(1,363,400)	(901,200)	(1,085,000)
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	(1,363,400)	(901,200)	(1,085,000)
Total Assets (as of)	(21,492,400)	(17,862,900)
Cash Flows Used for Investments in Plant	$ 0	$ 0	$ 0