REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,439,700	$ 4,526,300
Less: Current Portion	875,037	887,625
Total Long-Term Regulatory Assets	3,564,700	3,638,688
Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,782,500	1,817,800
Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	416,300	423,300
Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	645,400	644,500
Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	375,800	385,300
Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	459,300	464,400
Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	578,900	576,600
Contractusl Obligations - Yankee Companies [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	70,600	84,900
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	110,900	129,500
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,737,600	1,727,100
Less: Current Portion	370,083	335,526
Total Long-Term Regulatory Assets	1,367,486	1,391,564
The Connecticut Light And Power Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	421,500	429,300
The Connecticut Light And Power Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	413,500	420,500
The Connecticut Light And Power Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	435,900	437,000
The Connecticut Light And Power Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	225,100	239,800
The Connecticut Light And Power Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light And Power Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	170,500	123,900
The Connecticut Light And Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	71,100	76,600
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,327,600	1,347,100
Less: Current Portion	276,476	289,400
Total Long-Term Regulatory Assets	1,051,090	1,057,746
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	429,900	438,600
NSTAR Electric Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	2,500	2,800
NSTAR Electric Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	90,400	89,700
NSTAR Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	118,500	112,500
NSTAR Electric Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	394,300	398,700
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	245,900	257,300
NSTAR Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	46,100	47,500
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	354,900	362,700
Less: Current Portion	111,274	117,240
Total Long-Term Regulatory Assets	243,643	245,525
Public Service Company Of New Hampshire [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	181,100	184,200
Public Service Company Of New Hampshire [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	23,400	24,200
Public Service Company Of New Hampshire [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	17,000	17,100
Public Service Company Of New Hampshire [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	101,800	104,500
Public Service Company Of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	31,600	32,700
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	189,300	191,400
Less: Current Portion	66,033	64,123
Total Long-Term Regulatory Assets	123,317	127,291
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	85,100	86,700
Western Massachusetts Electric Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Western Massachusetts Electric Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	30,900	30,800
Western Massachusetts Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	15,200	15,900
Western Massachusetts Electric Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Western Massachusetts Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	49,100	46,700
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 9,000	$ 11,300