COMMON SHARES - Narrative (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Issued (in shares)	333,878,402	333,878,402
Common shares held as treasury shares (in shares)		16,992,594
Common shares repurchased (in shares)	321,228
Weighted average treasury stock (in dollars per share)	$ 24.26
Common Shares
Class of Stock [Line Items]
Common shares outstanding (in shares)	316,885,808	316,885,808	317,191,249	316,983,337
Treasury Stock | Eversource
Class of Stock [Line Items]
Share price of common shares repurchased (in dollars per share)	$ 52.56
Western Massachusetts Electric Company
Class of Stock [Line Items]
Issued (in shares)		100
NSTAR Electric Company
Class of Stock [Line Items]
Issued (in shares)	200	200
NSTAR Electric Company | Common Shares
Class of Stock [Line Items]
Common shares outstanding (in shares)	200	200	200	200