SEGMENT INFORMATION - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating Revenues	$ 2,034,900	$ 2,271,400	$ 1,853,900	$ 2,288,000	$ 1,895,600	$ 1,988,500	$ 1,762,800	$ 2,105,100	$ 8,448,201	$ 7,751,952	$ 7,639,129
Depreciation and Amortization									(1,072,000)	(863,800)	(787,100)
Other Operating Expenses									(5,676,300)	(5,000,000)	(5,010,700)
Operating Income	400,000	466,000	391,400	442,500	443,700	495,300	448,200	501,000	1,699,930	1,888,249	1,841,274
Interest Expense									(498,805)	(421,755)	(400,961)
Interest Income									18,100	8,300	11,000
Other Income/(Loss), Net									110,300	99,700	53,500
Income Tax Benefit									(288,972)	(578,892)	(554,997)
Net Income	233,200	291,300	244,600	271,400	239,400	262,200	232,600	261,300	1,040,519	995,515	949,821
Net Income Attributable to Noncontrolling Interests									(7,519)	(7,519)	(7,519)
Net Income/(Loss) Attributable to Common Shareholders	231,300	$ 289,400	$ 242,800	$ 269,500	237,400	$ 260,400	$ 230,700	$ 259,500	1,033,000	987,996	942,302
Total Assets (as of)	38,241,256				36,220,386				38,241,256	36,220,386
Cash Flows Used for Investments in Plant									2,523,371	2,348,105	1,976,867
Operating Segments | Eversource Electric Distribution
Segment Reporting Information [Line Items]
Operating Revenues									6,957,200	5,542,900	5,594,300
Depreciation and Amortization									(671,800)	(542,600)	(504,700)
Other Operating Expenses									(5,548,600)	(4,072,600)	(4,173,000)
Operating Income									736,800	927,700	916,600
Interest Expense									(202,800)	(186,300)	(193,100)
Interest Income									18,700	7,300	10,000
Other Income/(Loss), Net									67,500	41,600	22,700
Income Tax Benefit									(160,200)	(288,300)	(288,800)
Net Income									460,000	502,000	467,400
Net Income Attributable to Noncontrolling Interests									(4,600)	(4,600)	(4,600)
Net Income/(Loss) Attributable to Common Shareholders									455,400	497,400	462,800
Total Assets (as of)	21,389,100				19,250,400				21,389,100	19,250,400
Cash Flows Used for Investments in Plant									961,300	1,020,700	812,600
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Operating Revenues									1,022,200	947,300	857,700
Depreciation and Amortization									(75,000)	(72,900)	(65,300)
Other Operating Expenses									(787,600)	(716,400)	(629,000)
Operating Income									159,600	158,000	163,400
Interest Expense									(44,100)	(43,100)	(41,300)
Interest Income									0	100	100
Other Income/(Loss), Net									7,100	3,800	700
Income Tax Benefit									(29,400)	(44,200)	(45,200)
Net Income									93,200	74,600	77,700
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									93,200	74,600	77,700
Total Assets (as of)	3,904,900				3,595,200				3,904,900	3,595,200
Cash Flows Used for Investments in Plant									351,500	298,200	255,300
Operating Segments | Eversource Electric Transmission
Segment Reporting Information [Line Items]
Operating Revenues									1,286,300	1,301,700	1,210,000
Depreciation and Amortization									(231,800)	(209,400)	(185,800)
Other Operating Expenses									(375,500)	(382,800)	(321,300)
Operating Income									679,000	709,500	702,900
Interest Expense									(120,600)	(115,100)	(110,000)
Interest Income									2,400	1,800	1,200
Other Income/(Loss), Net									31,100	27,300	17,800
Income Tax Benefit									(161,800)	(228,700)	(238,200)
Net Income									430,100	394,800	373,700
Net Income Attributable to Noncontrolling Interests									(2,900)	(2,900)	(2,900)
Net Income/(Loss) Attributable to Common Shareholders									427,200	391,900	370,800
Total Assets (as of)	10,285,000				9,401,200				10,285,000	9,401,200
Cash Flows Used for Investments in Plant									929,700	867,600	801,000
Operating Segments | Eversource Water
Segment Reporting Information [Line Items]
Operating Revenues									212,000	15,900	0
Depreciation and Amortization									(46,500)	(3,700)	0
Other Operating Expenses									(99,800)	(8,300)	0
Operating Income									65,700	3,900	0
Interest Expense									(34,300)	(3,100)	0
Interest Income									0	100	0
Other Income/(Loss), Net									(400)	0	0
Income Tax Benefit									(100)	(2,100)	0
Net Income									30,900	(1,200)	0
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									30,900	(1,200)	0
Total Assets (as of)	2,253,000				2,182,900				2,253,000	2,182,900
Cash Flows Used for Investments in Plant									102,300	16,000	0
Operating Segments | Other Segments
Segment Reporting Information [Line Items]
Operating Revenues									936,300	931,000	870,400
Depreciation and Amortization									(49,100)	(37,400)	(33,500)
Other Operating Expenses									(831,500)	(806,600)	(779,200)
Operating Income									55,700	87,000	57,700
Interest Expense									(129,300)	(90,000)	(63,500)
Interest Income									30,300	15,700	7,000
Other Income/(Loss), Net									1,185,300	1,113,000	1,021,200
Income Tax Benefit									62,500	(15,500)	16,500
Net Income									1,204,500	1,110,200	1,038,900
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									1,204,500	1,110,200	1,038,900
Total Assets (as of)	17,874,200				16,220,900				17,874,200	16,220,900
Cash Flows Used for Investments in Plant									178,600	145,600	108,000
Eliminations
Segment Reporting Information [Line Items]
Operating Revenues									(1,965,800)	(986,800)	(893,300)
Depreciation and Amortization									2,200	2,200	2,200
Other Operating Expenses									1,966,700	986,700	891,800
Operating Income									3,100	2,100	700
Interest Expense									32,300	15,800	6,900
Interest Income									(33,300)	(16,700)	(7,300)
Other Income/(Loss), Net									(1,180,300)	(1,086,000)	(1,008,900)
Income Tax Benefit									0	(100)	700
Net Income									(1,178,200)	(1,084,900)	(1,007,900)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income/(Loss) Attributable to Common Shareholders									(1,178,200)	(1,084,900)	(1,007,900)
Total Assets (as of)	$ (17,464,900)				$ (14,430,200)				(17,464,900)	(14,430,200)
Cash Flows Used for Investments in Plant									$ 0	$ 0	$ 0