COMMON SHARES - Narrative (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common shares held as treasury shares (in shares)	16,992,594	16,671,366
Common shares repurchased (in shares)	321,228	532,521
Weighted average treasury stock (in dollars per share)	$ 24.26	$ 26.02
Treasury Stock [Member] | Eversource [Member]
Class of Stock [Line Items]
Share price of common shares repurchased (in dollars per share)	$ 52.56	$ 47.94