SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions		12 Months Ended
	Mar. 25, 2016 USD ($)	Dec. 31, 2016 USD ($) companies utility customer	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer		3.7
Goodwill		$ 3,519,401	$ 3,519,401
Available for sale securities subject to new accounting guidance		48,000
Impact of share based compensation accounting guidance on income tax		19,100
Impact of new accounting standard on cash flow statement		26,600	9,700	$ 16,500
Equity method investment		$ 236,900	48,000
Number of companies that transmit electricity imported from hydro-quebec system | companies		2
Number of nuclear generation companies | companies		3
Regulatory assets		$ 4,526,300	4,583,800
Eversource [Member]
Summary of Accounting Policies [Line Items]
Goodwill		3,231,811	3,231,811
Cash collateral deposited		21,700	17,100
Equity method investment		9,703,287	8,915,178
The Connecticut Light And Power Company [Member]
Summary of Accounting Policies [Line Items]
Cash collateral deposited		1,400	700
Damages paid			244,600
Accumulated interest from damages awarded			178,000
Long term receivable		25,000
Regulatory assets		1,727,100	1,637,300
NSTAR Electric Company [Member]
Summary of Accounting Policies [Line Items]
Cash collateral deposited		11,800	8,500
Regulatory assets		1,347,100	1,461,400
Public Service Company Of New Hampshire [Member]
Summary of Accounting Policies [Line Items]
Cash collateral deposited		500	1,500
Long term receivable		3,800
Regulatory assets		362,700	362,900
Western Massachusetts Electric Company [Member]
Summary of Accounting Policies [Line Items]
Damages paid			57,400
Accumulated interest from damages awarded			41,800
Long term receivable		5,500
Regulatory assets		$ 191,400	191,200
Connecticut, Massachusetts and New Hampshire [Member]
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility		6
Algonquin Gas Tansmsission LLC - Access Northeast [Member]
Summary of Accounting Policies [Line Items]
Equity method investment		$ 30,900	$ 10,700
Equity method investment, ownership percentage		40.00%	40.00%
FERC Regulated Transmission Business [Member]
Summary of Accounting Policies [Line Items]
Equity method investment		$ 154,600
Equity method investment, ownership percentage		15.00%
Hydro-Quebec System [Member]
Summary of Accounting Policies [Line Items]
Equity method investment		$ 7,700	$ 7,000
Equity method investment, ownership percentage		37.20%	37.20%
Hydro-Quebec System [Member] | NSTAR Electric Company [Member]
Summary of Accounting Policies [Line Items]
Equity method investment		$ 3,000	$ 2,700
Equity method investment, ownership percentage		14.50%	14.50%
Yankee Companies [Member]
Summary of Accounting Policies [Line Items]
Net of damages awarded		$ 52,200
CYAPC and YAEC [Member]
Summary of Accounting Policies [Line Items]
Net of damages awarded	$ 32,600
Total amount of damages awarded		6,800
MYAPC [Member]
Summary of Accounting Policies [Line Items]
Net of damages awarded	$ 24,600	$ 14,400
Maximum [Member]
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress		180 days
Minimum [Member]
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress		90 days
Other Equity Method Investments [Member]
Summary of Accounting Policies [Line Items]
Equity method investment		$ 43,700	$ 30,300