LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Nov. 18, 2016	May 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt outstanding	$ 8,829,354				$ 8,805,574
Less Amounts due Within One Year	(773,883)				(228,883)
Senior Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	8,829,400				8,805,600
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	2,516,010				2,763,682
Less Amounts due Within One Year	(250,000)				0
The Connecticut Light And Power Company [Member] | Fixed rate Tax Exempt Due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	120,500				120,500
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	2,669,800				2,669,800
Unamortized Premiums and Discounts, Net	(10,000)				(10,700)
Unamortized Debt Issuance Costs	(14,300)				(15,900)
Long-Term Debt	$ 2,516,000				$ 2,763,700
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series D 1994 Notes Due 2024 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	7.875%				7.875%
Long-term debt outstanding	$ 139,800				$ 139,800
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series B 2004 Notes Due 2034 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.75%				5.75%
Long-term debt outstanding	$ 130,000				$ 130,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series B 2005 Notes Due 2035 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.625%				5.625%
Long-term debt outstanding	$ 100,000				$ 100,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2006 Notes Due 2036 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.35%				6.35%
Long-term debt outstanding	$ 250,000				$ 250,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2007 Notes Due 2017 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.375%				5.375%
Long-term debt outstanding	$ 150,000				$ 150,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series B 2007 Notes Due 2037 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.75%				5.75%
Long-term debt outstanding	$ 150,000				$ 150,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series C 2007 Notes Due 2017 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.75%				5.75%
Long-term debt outstanding	$ 100,000				$ 100,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series D 2007 Notes Due 2037 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.375%				6.375%
Long-term debt outstanding	$ 100,000				$ 100,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2008 Notes Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.65%				5.65%
Long-term debt outstanding	$ 300,000				$ 300,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2009 Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.50%				5.50%
Long-term debt outstanding	$ 250,000				$ 250,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2013 Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.50%				2.50%
Long-term debt outstanding	$ 400,000				$ 400,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2014 Notes Due 2044 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.30%				4.30%
Long-term debt outstanding	$ 250,000				$ 250,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Series A 2015 Notes Due 2045 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.15%				4.15%
Long-term debt outstanding	$ 350,000				$ 350,000
The Connecticut Light And Power Company [Member] | First Mortgage Notes [Member] | Fixed rate Tax Exempt Due 2028 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.375%				4.375%
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	$ 1,678,116	$ 700,000			$ 1,829,766
Less Amounts due Within One Year	(400,000)				(200,000)
NSTAR Electric Company [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	2,100,000				2,050,000
Less Amounts due Within One Year	(400,000)				(200,000)
Unamortized Premiums and Discounts, Net	(9,100)				(8,500)
Unamortized Debt Issuance Costs	(12,800)				(11,700)
Long-Term Debt	$ 1,678,100				$ 1,829,800
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2036 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.75%				5.75%
Long-term debt outstanding	$ 200,000				$ 200,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2017 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.625%				5.625%
Long-term debt outstanding	$ 400,000				$ 400,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2040 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.50%				5.50%
Long-term debt outstanding	$ 300,000				$ 300,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2022 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.375%				2.375%
Long-term debt outstanding	$ 400,000				$ 400,000
NSTAR Electric Company [Member] | Debentures [Member] | Variable Rate Due 2016 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)					0.6036%
Long-term debt outstanding	$ 0				$ 200,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2044 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.40%				4.40%
Long-term debt outstanding	$ 300,000				$ 300,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2025 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.25%				3.25%
Long-term debt outstanding	$ 250,000				$ 250,000
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.70%		2.70%		2.70%
Long-term debt outstanding	$ 250,000				$ 0
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	1,002,048				1,071,017
Less Amounts due Within One Year	(70,000)				0
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	987,000				987,000
Less Amounts due Within One Year	(70,000)				0
Unamortized Premiums and Discounts, Net	100				100
Unamortized Debt Issuance Costs	(4,400)				(5,400)
Long-Term Debt	$ 1,002,000				$ 1,071,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series M Notes Due 2035 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.60%				5.60%
Long-term debt outstanding	$ 50,000				$ 50,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series N Notes Due 2017 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.15%				6.15%
Long-term debt outstanding	$ 70,000				$ 70,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series O Notes Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.00%				6.00%
Long-term debt outstanding	$ 110,000				$ 110,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series P Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.50%				4.50%
Long-term debt outstanding	$ 150,000				$ 150,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series Q Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.05%				4.05%
Long-term debt outstanding	$ 122,000				$ 122,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series R Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.20%				3.20%
Long-term debt outstanding	$ 160,000				$ 160,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Series S Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.50%				3.50%
Long-term debt outstanding	$ 325,000				$ 325,000
Public Service Company Of New Hampshire [Member] | First Mortgage Notes [Member] | Adjustable Rate Tax Exempt Series A Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	1.138%				0.193%
Long-term debt outstanding	$ 89,300				$ 89,300
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	566,536				517,329
Western Massachusetts Electric Company [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	565,000				515,000
Unamortized Premiums and Discounts, Net	4,200				5,200
Unamortized Debt Issuance Costs	(2,700)				(2,900)
Long-Term Debt	$ 566,500				$ 517,300
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series B Notes Due 2034 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.90%				5.90%
Long-term debt outstanding	$ 50,000				$ 50,000
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series D Notes Due 2037 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.70%				6.70%
Long-term debt outstanding	$ 40,000				$ 40,000
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series E Notes Due 2020 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.10%				5.10%
Long-term debt outstanding	$ 95,000				$ 95,000
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series F Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.50%				3.50%
Long-term debt outstanding	$ 250,000				$ 250,000
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series G Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.88%				3.88%
Long-term debt outstanding	$ 80,000				$ 80,000
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series H Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.75%				2.75%
Long-term debt outstanding	$ 50,000				$ 0
Yankee Gas Services Company [Member] | First Mortgage Notes [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	445,000				445,000
Unamortized Premiums and Discounts, Net	400				400
Unamortized Debt Issuance Costs	(1,500)				(1,700)
Long-Term Debt	$ 443,900				$ 443,700
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series B Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	8.48%				8.48%
Long-term debt outstanding	$ 20,000				$ 20,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series H Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.26%				5.26%
Long-term debt outstanding	$ 50,000				$ 50,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series I Senior Notes Due 2035 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.35%				5.35%
Long-term debt outstanding	$ 50,000				$ 50,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series J Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.90%				6.90%
Long-term debt outstanding	$ 100,000				$ 100,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series K Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.87%				4.87%
Long-term debt outstanding	$ 50,000				$ 50,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series L Senior Notes Due 2044 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.82%				4.82%
Long-term debt outstanding	$ 100,000				$ 100,000
Yankee Gas Services Company [Member] | First Mortgage Notes [Member] | Series M Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.35%				3.35%
Long-term debt outstanding	$ 75,000				$ 75,000
NSTAR Gas Company [Member] | First Mortgage Notes [Member]
Debt Instrument [Line Items]
Less Amounts due Within One Year	(25,000)				0
Unamortized Debt Issuance Costs	(700)				(800)
Long-Term Debt	284,300				309,200
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	$ 310,000				$ 310,000
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member] | Series J Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	9.95%				9.95%
Long-term debt outstanding	$ 25,000				$ 25,000
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member] | Series K Senior Notes Due 2033 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	7.11%				7.11%
Long-term debt outstanding	$ 35,000				$ 35,000
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member] | Series M Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	7.04%				7.04%
Long-term debt outstanding	$ 25,000				$ 25,000
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member] | Series N Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.46%				4.46%
Long-term debt outstanding	$ 125,000				$ 125,000
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member] | Series O Senior Notes Due 2045 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.35%				4.35%
Long-term debt outstanding	$ 100,000				$ 100,000
Eversource [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	2,158,519				1,691,330
Less Amounts due Within One Year	$ (28,883)				$ (28,883)
Eversource [Member] | Debentures [Member] | Unsecured Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.50%				4.50%
Long-term debt outstanding	$ 350,000				$ 350,000
Eversource [Member] | Debentures [Member] | Series E Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	1.45%				1.45%
Eversource [Member] | Debentures [Member] | Series F Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.80%				2.80%
Eversource [Member] | Debentures [Member] | Series G Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	1.60%				1.60%
Eversource [Member] | Debentures [Member] | Series H Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.15%				3.15%
Eversource [Member] | Debentures [Member] | Series I Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.50%				2.50%
Eversource [Member] | Debentures [Member] | Series J Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.35%				3.35%
Eversource [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized Premiums and Discounts, Net	$ (2,200)				$ (1,300)
Unamortized Debt Issuance Costs	(4,900)				(1,900)
Eversource [Member] | Senior Notes [Member] | Series E Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	300,000				300,000
Eversource [Member] | Senior Notes [Member] | Series F Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	450,000				450,000
Eversource [Member] | Senior Notes [Member] | Series G Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	150,000				150,000
Eversource [Member] | Senior Notes [Member] | Series H Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	300,000				300,000
Eversource [Member] | Senior Notes [Member] | Series I Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)				2.50%
Long-term debt outstanding	250,000				0
Eversource [Member] | Senior Notes [Member] | Series J Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)				3.35%
Long-term debt outstanding	250,000				0
Eversource [Member] | Senior Notes and Debentures [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	2,050,000				1,550,000
Eversource [Member] | Spent Nuclear Fuel Obilgation CY [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	180,000				179,500
Eversource [Member] | Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	144,600				173,500
Eversource [Member] | Fair Value Adjustment - Current Portion [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	(28,900)				(28,900)
Eversource [Member] | Total Other Long-Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	$ 3,066,800				$ 2,623,800