REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,252,000	$ 4,526,300
Less: Current Portion	746,142	887,625
Total Long-Term Regulatory Assets	3,505,901	3,638,688
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,793,800	1,817,800
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	385,100	423,300
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	652,700	644,500
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	330,100	385,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	449,000	464,400
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	470,700	576,600
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	104,800	99,300
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	65,800	115,100
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,587,200	1,727,100
Less: Current Portion	274,982	335,526
Total Long-Term Regulatory Assets	1,312,191	1,391,564
The Connecticut Light And Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	415,800	429,300
The Connecticut Light And Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	381,600	420,500
The Connecticut Light And Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	441,100	437,000
The Connecticut Light And Power Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	195,700	239,800
The Connecticut Light And Power Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	87,900	123,900
The Connecticut Light And Power Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	35,100	33,200
The Connecticut Light And Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	30,000	43,400
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,279,900	1,347,100
Less: Current Portion	230,620	289,400
Total Long-Term Regulatory Assets	1,049,324	1,057,746
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	436,700	438,600
NSTAR Electric Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	2,400	2,800
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	92,400	89,700
NSTAR Electric Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	112,400	112,500
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	385,500	398,700
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	201,100	257,300
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	33,900	31,900
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	15,500	15,600
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	357,100	362,700
Less: Current Portion	112,465	117,240
Total Long-Term Regulatory Assets	244,561	245,525
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	183,200	184,200
Public Service Company Of New Hampshire | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	22,300	24,200
Public Service Company Of New Hampshire | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	9,200	17,100
Public Service Company Of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	108,000	104,500
Public Service Company Of New Hampshire | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	16,800	16,200
Public Service Company Of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	17,600	16,500
Western Massachusetts Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	182,400	191,400
Less: Current Portion	60,606	64,123
Total Long-Term Regulatory Assets	121,796	127,291
Western Massachusetts Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	84,800	86,700
Western Massachusetts Electric Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Western Massachusetts Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	30,500	30,800
Western Massachusetts Electric Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	12,800	15,900
Western Massachusetts Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Western Massachusetts Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	44,400	46,700
Western Massachusetts Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	4,500	4,200
Western Massachusetts Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,400	$ 7,100