REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 4,451,400	$ 4,526,300
Less: Current Portion	870,393	887,625
Total Long-Term Regulatory Assets	3,580,981	3,638,688
Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,833,600	1,817,800
Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	398,100	423,300
Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	652,500	644,500
Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	352,900	385,300
Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	454,200	464,400
Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	584,300	576,600
Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	103,000	99,300
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	72,800	115,100
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,689,500	1,727,100
Less: Current Portion	346,520	335,526
Total Long-Term Regulatory Assets	1,343,011	1,391,564
The Connecticut Light And Power Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	423,500	429,300
The Connecticut Light And Power Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	394,800	420,500
The Connecticut Light And Power Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	440,700	437,000
The Connecticut Light And Power Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	210,300	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	153,000	123,900
The Connecticut Light And Power Company [Member] | Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	34,500	33,200
The Connecticut Light And Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	32,700	43,400
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	1,334,300	1,347,100
Less: Current Portion	270,230	289,400
Total Long-Term Regulatory Assets	1,064,099	1,057,746
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	445,600	438,600
NSTAR Electric Company [Member] | Derivative Liabilities [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	2,300	2,800
NSTAR Electric Company [Member] | Income Taxes, Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	91,200	89,700
NSTAR Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	115,300	112,500
NSTAR Electric Company [Member] | Goodwill-related [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	389,900	398,700
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	240,700	257,300
NSTAR Electric Company [Member] | Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	33,200	31,900
NSTAR Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	16,100	15,600
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	382,000	362,700
Less: Current Portion	132,424	117,240
Total Long-Term Regulatory Assets	249,612	245,525
Public Service Company Of New Hampshire [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	189,700	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	22,400	24,200
Public Service Company Of New Hampshire [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	12,900	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	125,700	104,500
Public Service Company Of New Hampshire [Member] | Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	16,600	16,200
Public Service Company Of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	14,700	16,500
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	192,200	191,400
Less: Current Portion	67,310	64,123
Total Long-Term Regulatory Assets	124,899	127,291
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	86,400	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	31,200	30,800
Western Massachusetts Electric Company [Member] | Storm Restoration Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	14,400	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	50,500	46,700
Western Massachusetts Electric Company [Member] | Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	4,400	4,200
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,300	$ 7,100