REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,919,500	$ 5,145,900
Less: Current Portion	515,265	514,779
Total Long-Term Regulatory Assets	4,404,176	4,631,137
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,870,500	1,914,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	702,600	728,600
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	586,900	608,400
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	536,700	576,000
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	289,000	316,000
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	347,900	356,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	340,000	348,400
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	97,100	89,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	148,800	208,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,608,500	1,630,700
Less: Current Portion	175,706	125,155
Total Long-Term Regulatory Assets	1,432,827	1,505,488
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	407,400	424,700
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	456,500	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	279,600	302,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	60,300	33,200
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	345,400	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	33,300	32,300
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	26,000	27,000
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,389,000	1,438,200
Less: Current Portion	210,032	241,747
Total Long-Term Regulatory Assets	1,178,974	1,196,512
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	554,800	544,400
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	104,100	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	191,200	212,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	138,200	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	291,900	299,100
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	48,300	42,200
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	60,500	64,600
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	905,800	929,500
Less: Current Portion	77,854	67,228
Total Long-Term Regulatory Assets	827,898	862,288
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	151,100	169,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	10,100	8,300
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	586,900	608,400
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	65,900	60,500
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	70,600	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	3,500	3,300
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 17,700	$ 12,100