GOODWILL - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012	Dec. 31, 2000
Goodwill [Line Items]
Goodwill related to the acquisition of Yankee Gas	$ 3,519,401,000	$ 3,519,401,000	$ 3,519,401,000
Goodwill impairment	$ 0
Changes in goodwill balance		$ 0	$ 0
NSTAR Electric Company [Member]
Goodwill [Line Items]
Goodwill acquired during period				$ 3,200,000,000
Yankee Gas Services Company [Member]
Goodwill [Line Items]
Goodwill related to the acquisition of Yankee Gas					$ 300,000,000