COMMON SHARES - (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	333,878,402
Treasury stock (in shares)	16,992,594	16,992,594
The Connecticut Light And Power Company
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
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)	200	200
Public Service Company Of New Hampshire
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
Common Shares
Class of Stock [Line Items]
Common stock outstanding (in shares)	316,885,808	316,885,808	316,885,808	317,191,249
Common Shares | The Connecticut Light And Power Company
Class of Stock [Line Items]
Common stock outstanding (in shares)	6,035,205	6,035,205	6,035,205	6,035,205
Common Shares | NSTAR Electric Company
Class of Stock [Line Items]
Common stock outstanding (in shares)	200	200	200	200
Common Shares | Public Service Company Of New Hampshire
Class of Stock [Line Items]
Common stock outstanding (in shares)	301	301	301	301