SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Revenues	$ 9,863,085	$ 8,904,430	$ 8,526,470
Depreciation and Amortization	(1,335,000)	(1,159,100)	(1,080,700)
Impairment of Northern Pass Transmission	0	0	(239,644)
Other Operating Expenses	(6,534,800)	(5,756,600)	(5,615,700)
Operating Income	1,993,321	1,988,734	1,590,491
Interest Expense	(582,334)	(538,452)	(533,197)
Interest Income	25,600	4,800	12,800
Other Income, Net	135,700	103,800	120,000
Income Tax (Expense)/Benefit	(344,223)	(346,186)	(273,499)
Net Income	1,228,046	1,212,686	916,572
Net Income Attributable to Noncontrolling Interests	(7,519)	(7,519)	(7,519)
Net Income Attributable to Common Shareholders	1,220,527	1,205,167	909,053
Total Assets (as of)	48,492,144	46,099,598
Cash Flows Used for Investments in Plant	3,175,080	2,942,996	2,911,489
Operating Segments | Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues	7,423,600	7,132,300	6,976,500
Depreciation and Amortization	(737,800)	(657,000)	(651,300)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(5,970,000)	(5,642,300)	(5,525,100)
Operating Income	715,800	833,000	800,100
Interest Expense	(236,400)	(216,000)	(206,400)
Interest Income	20,700	3,200	13,300
Other Income, Net	78,100	58,000	46,800
Income Tax (Expense)/Benefit	(103,500)	(129,600)	(135,900)
Net Income	474,700	548,600	517,900
Net Income Attributable to Noncontrolling Interests	(4,600)	(4,600)	(4,600)
Net Income Attributable to Common Shareholders	470,100	544,000	513,300
Total Assets (as of)	25,411,200	24,981,900
Cash Flows Used for Investments in Plant	1,053,300	1,079,000	1,104,200
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues	1,789,600	1,208,700	1,062,200
Depreciation and Amortization	(142,300)	(87,900)	(68,300)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(1,345,400)	(913,800)	(830,800)
Operating Income	301,900	207,000	163,100
Interest Expense	(58,600)	(40,000)	(47,400)
Interest Income	4,500	900	100
Other Income, Net	17,900	3,100	1,600
Income Tax (Expense)/Benefit	(60,900)	(36,900)	(21,200)
Net Income	204,800	134,100	96,200
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	204,800	134,100	96,200
Total Assets (as of)	7,215,900	6,450,500
Cash Flows Used for Investments in Plant	721,100	494,400	460,200
Operating Segments | Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues	1,634,600	1,536,100	1,389,000
Depreciation and Amortization	(300,300)	(278,100)	(253,300)
Impairment of Northern Pass Transmission			(239,600)
Other Operating Expenses	(496,200)	(470,000)	(411,200)
Operating Income	838,100	788,000	484,900
Interest Expense	(133,200)	(126,800)	(125,700)
Interest Income	2,200	4,700	1,500
Other Income, Net	19,800	23,300	29,200
Income Tax (Expense)/Benefit	(179,400)	(183,800)	(130,500)
Net Income	547,500	505,400	259,400
Net Income Attributable to Noncontrolling Interests	(2,900)	(2,900)	(2,900)
Net Income Attributable to Common Shareholders	544,600	502,500	256,500
Total Assets (as of)	12,377,800	11,695,000
Cash Flows Used for Investments in Plant	1,024,100	1,004,600	987,000
Operating Segments | Water Distribution
Segment Reporting Information [Line Items]
Operating Revenues	211,300	215,400	214,600
Depreciation and Amortization	(46,100)	(44,200)	(46,900)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(101,400)	(86,600)	(101,000)
Operating Income	63,800	84,600	66,700
Interest Expense	(32,000)	(32,900)	(34,600)
Interest Income	0	0	0
Other Income, Net	3,300	2,000	400
Income Tax (Expense)/Benefit	1,700	(12,500)	2,400
Net Income	36,800	41,200	34,900
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	36,800	41,200	34,900
Total Assets (as of)	2,551,100	2,375,200
Cash Flows Used for Investments in Plant	137,200	118,800	118,000
Other
Segment Reporting Information [Line Items]
Operating Revenues	1,354,000	1,235,900	1,028,500
Depreciation and Amortization	(113,100)	(93,500)	(63,200)
Impairment of Northern Pass Transmission			0
Other Operating Expenses	(1,170,400)	(1,071,900)	(891,300)
Operating Income	70,500	70,500	74,000
Interest Expense	(168,800)	(161,000)	(170,300)
Interest Income	46,000	37,800	48,700
Other Income, Net	1,363,900	1,382,900	945,300
Income Tax (Expense)/Benefit	(2,100)	16,600	11,700
Net Income	1,309,500	1,346,800	909,400
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	1,309,500	1,346,800	909,400
Total Assets (as of)	22,674,700	22,089,400
Cash Flows Used for Investments in Plant	239,400	246,200	242,100
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(2,550,000)	(2,424,000)	(2,144,300)
Depreciation and Amortization	4,600	1,600	2,300
Impairment of Northern Pass Transmission			0
Other Operating Expenses	2,548,600	2,428,000	2,143,700
Operating Income	3,200	5,600	1,700
Interest Expense	46,600	38,300	51,200
Interest Income	(47,800)	(41,800)	(50,800)
Other Income, Net	(1,347,300)	(1,365,500)	(903,300)
Income Tax (Expense)/Benefit	0	0	0
Net Income	(1,345,300)	(1,363,400)	(901,200)
Net Income Attributable to Noncontrolling Interests	0	0	0
Net Income Attributable to Common Shareholders	(1,345,300)	(1,363,400)	(901,200)
Total Assets (as of)	(21,738,600)	(21,492,400)
Cash Flows Used for Investments in Plant	$ 0	$ 0	$ 0