REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,701,400	$ 5,715,800
Less:  Current Portion	1,128,088	1,129,093
Total Long-Term Regulatory Assets	4,573,309	4,586,709
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,452,500	1,481,000
Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	1,249,300	1,102,700
Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	937,700	1,050,500
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	791,200	790,700
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	468,100	478,900
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	293,600	297,800
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	229,400	249,200
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	117,500	115,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	162,100	150,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	2,063,200	2,084,800
Less:  Current Portion	382,112	371,609
Total Long-Term Regulatory Assets	1,681,057	1,713,161
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	268,000	272,400
CL&P | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	735,700	695,600
CL&P | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	295,200	333,600
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	471,800	470,500
CL&P | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	229,400	249,200
CL&P | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	34,200	33,600
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	28,900	29,900
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,673,500	1,579,200
Less:  Current Portion	433,653	443,956
Total Long-Term Regulatory Assets	1,239,773	1,135,231
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	386,900	395,500
NSTAR Electric | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	449,400	341,300
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	375,000	376,600
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	112,800	112,600
NSTAR Electric | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	252,100	255,700
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	61,900	59,800
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	35,400	37,700
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	779,200	786,400
Less:  Current Portion	124,701	107,169
Total Long-Term Regulatory Assets	654,507	679,182
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	116,500	118,900
PSNH | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	64,200	65,800
PSNH | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	97,000	85,400
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	14,500	17,500
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	468,100	478,900
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	4,200	4,100
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 14,700	$ 15,800