REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,830,800	$ 5,145,900
Less: Current Portion	538,162	514,779
Total Long-Term Regulatory Assets	4,292,560	4,631,137
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,835,100	1,914,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	713,400	728,600
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	576,100	608,400
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	508,900	576,000
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	282,700	316,000
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	345,800	356,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	335,700	348,400
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	101,300	89,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	131,800	208,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,577,800	1,630,700
Less: Current Portion	173,339	125,155
Total Long-Term Regulatory Assets	1,404,502	1,505,488
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	400,500	424,700
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	460,200	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	258,900	302,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	56,500	33,200
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	342,300	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	33,800	32,300
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	25,600	27,000
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,360,300	1,438,200
Less: Current Portion	201,215	241,747
Total Long-Term Regulatory Assets	1,159,056	1,196,512
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	542,800	544,400
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	108,600	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	190,200	212,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	121,100	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	288,300	299,100
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	51,700	42,200
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	57,600	64,600
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	870,400	929,500
Less: Current Portion	78,097	67,228
Total Long-Term Regulatory Assets	792,332	862,288
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	148,500	169,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	11,400	8,300
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	576,100	608,400
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	59,800	60,500
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	58,100	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	$ 13,000	$ 12,100