REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,096,700	$ 5,145,900
Less: Current Portion	507,255	514,779
Total Long-Term Regulatory Assets	4,589,427	4,631,137
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,878,700	1,914,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	730,900	728,600
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	597,700	608,400
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	579,000	576,000
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	317,700	316,000
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	353,500	356,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	344,200	348,400
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	92,800	89,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	202,200	208,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,644,400	1,630,700
Less: Current Portion	147,712	125,155
Total Long-Term Regulatory Assets	1,496,679	1,505,488
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	416,000	424,700
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	454,300	454,400
CL&P | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	307,400	302,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	54,400	33,200
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	353,100	356,500
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	32,800	32,300
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	26,400	27,000
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,408,600	1,438,200
Less: Current Portion	234,662	241,747
Total Long-Term Regulatory Assets	1,173,851	1,196,512
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	534,900	544,400
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	106,100	105,900
NSTAR Electric | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	204,400	212,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	160,200	169,100
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	295,500	299,100
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	44,900	42,200
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	62,600	64,600
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	932,000	929,500
Less: Current Portion	76,599	67,228
Total Long-Term Regulatory Assets	855,369	862,288
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	165,800	169,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	8,000	8,300
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	597,700	608,400
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	67,200	60,500
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	78,000	67,300
PSNH | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	3,400	3,300
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 11,900	$ 12,100