PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 15,245,900	$ 15,071,100
Distribution - Natural Gas	3,635,600	3,546,200
Transmission - Electric	10,251,100	10,153,900
Distribution - Water	1,649,500	1,639,800
Solar	169,900	164,100
Utility	30,952,000	30,575,100
Other	798,300	778,600
Property, Plant and Equipment, Gross	31,750,300	31,353,700
Less: Accumulated Depreciation
Utility	(7,229,500)	(7,126,200)
Other	(348,700)	(336,700)
Total Accumulated Depreciation	(7,578,200)	(7,462,900)
Property, Plant and Equipment, Net	24,172,100	23,890,800
Construction Work in Progress	1,860,700	1,719,600
Net Plant	26,032,781	25,610,428
CL&P
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,255,800	6,176,400
Transmission - Electric	4,746,400	4,700,500
Solar	0	0
Property, Plant and Equipment, Gross	11,002,200	10,876,900
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,328,000)	(2,302,600)
Property, Plant and Equipment, Net	8,674,200	8,574,300
Construction Work in Progress	391,700	335,400
Net Plant	9,065,880	8,909,701
NSTAR Electric
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,828,500	6,756,400
Transmission - Electric	4,085,600	4,065,900
Solar	169,900	164,100
Property, Plant and Equipment, Gross	11,084,000	10,986,400
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,749,900)	(2,702,000)
Property, Plant and Equipment, Net	8,334,100	8,284,400
Construction Work in Progress	581,500	510,300
Net Plant	8,915,608	8,794,700
PSNH
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	2,201,800	2,178,600
Transmission - Electric	1,370,200	1,338,700
Solar	0	0
Property, Plant and Equipment, Gross	3,572,000	3,517,300
Less: Accumulated Depreciation
Total Accumulated Depreciation	(789,300)	(772,900)
Property, Plant and Equipment, Net	2,782,700	2,744,400
Construction Work in Progress	137,900	135,700
Net Plant	$ 2,920,560	$ 2,880,073