REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,222,300	$ 4,583,800
Less: Current Portion	752,378	845,843
Total Long-Term Regulatory Assets	3,469,879	3,737,960
Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,618,700	1,828,200
Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	443,700	388,000
Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	641,700	650,900
Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	407,700	436,900
Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	469,500	484,900
Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	454,100	526,500
Contractusl Obligations - Yankee Companies [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	66,000	134,400
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	120,900	134,000
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,666,300	1,637,300
Less: Current Portion	267,733	268,318
Total Long-Term Regulatory Assets	1,398,610	1,369,028
The Connecticut Light And Power Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	407,100	413,600
The Connecticut Light And Power Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	441,000	380,800
The Connecticut Light And Power Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	438,500	444,400
The Connecticut Light And Power Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	253,700	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	50,600	45,100
The Connecticut Light And Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	75,400	82,000
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,302,400	1,461,400
Less: Current Portion	259,213	348,408
Total Long-Term Regulatory Assets	1,043,237	1,112,977
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	414,600	479,900
NSTAR Electric Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	2,700	1,300
NSTAR Electric Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	88,300	85,700
NSTAR Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	115,100	110,900
NSTAR Electric Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	403,100	416,300
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	230,400	311,000
NSTAR Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	48,200	56,300
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	353,200	362,900
Less: Current Portion	100,598	104,971
Total Long-Term Regulatory Assets	252,609	257,873
Public Service Company Of New Hampshire [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	174,700	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	27,000	34,500
Public Service Company Of New Hampshire [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	20,800	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	99,900	101,200
Public Service Company Of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	30,800	31,500
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	182,400	191,200
Less: Current Portion	54,081	56,166
Total Long-Term Regulatory Assets	128,308	135,010
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	82,100	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	31,200	31,800
Western Massachusetts Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	18,100	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	37,800	40,100
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 13,200	$ 11,300