COMMON SHARES - Schedule of Common Shares Authorized and Issued (Details) - $ / shares	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Per Share Par Value (in dollars per share)	$ 5	$ 5	$ 5
Authorized (in shares)	380,000,000	380,000,000
Issued (in shares)	333,878,402	333,862,615
The Connecticut Light And Power Company [Member]
Class of Stock [Line Items]
Per Share Par Value (in dollars per share)	$ 10	$ 10
Authorized (in shares)	24,500,000	24,500,000
Issued (in shares)	6,035,205	6,035,205
NSTAR Electric Company [Member]
Class of Stock [Line Items]
Per Share Par Value (in dollars per share)	$ 1	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	100	100
Public Service Company Of New Hampshire [Member]
Class of Stock [Line Items]
Per Share Par Value (in dollars per share)	$ 1	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	301	301
Western Massachusetts Electric Company [Member]
Class of Stock [Line Items]
Per Share Par Value (in dollars per share)	$ 25	$ 25
Authorized (in shares)	1,072,471	1,072,471
Issued (in shares)	434,653	434,653