REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,526,300	$ 4,583,800
Less: Current Portion	887,625	845,843
Total Long-Term Regulatory Assets	3,638,688	3,737,960
Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,817,800	1,828,200
Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	423,300	388,000
Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	644,500	650,900
Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	385,300	436,900
Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	464,400	484,900
Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	576,600	526,500
Contractual Obligations - Yankee Companies [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	84,900	134,400
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	129,500	134,000
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,727,100	1,637,300
Less: Current Portion	335,526	268,318
Total Long-Term Regulatory Assets	1,391,564	1,369,028
The Connecticut Light And Power Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	429,300	413,600
The Connecticut Light And Power Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	420,500	380,800
The Connecticut Light And Power Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	437,000	444,400
The Connecticut Light And Power Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	239,800	271,400
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	123,900	45,100
The Connecticut Light And Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	76,600	82,000
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,347,100	1,461,400
Less: Current Portion	289,400	348,408
Total Long-Term Regulatory Assets	1,057,746	1,112,977
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	438,600	479,900
NSTAR Electric Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	2,800	1,300
NSTAR Electric Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	89,700	85,700
NSTAR Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	112,500	110,900
NSTAR Electric Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	398,700	416,300
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	257,300	311,000
NSTAR Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	47,500	56,300
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	362,700	362,900
Less: Current Portion	117,240	104,971
Total Long-Term Regulatory Assets	245,525	257,873
Public Service Company Of New Hampshire [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	184,200	164,200
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	24,200	34,500
Public Service Company Of New Hampshire [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	17,100	31,500
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	104,500	101,200
Public Service Company Of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	32,700	31,500
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	191,400	191,200
Less: Current Portion	64,123	56,166
Total Long-Term Regulatory Assets	127,291	135,010
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	86,700	84,900
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	30,800	31,800
Western Massachusetts Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	15,900	23,100
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	46,700	40,100
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 11,300	$ 11,300