SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Statement of Income Captions [Line Items]
Net Income	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 183,700	$ 237,800	$ 209,400	$ 255,100	$ 949,821	$ 886,004	$ 827,065
Other Comprehensive Income/(Loss), Net of Tax:
Qualified Cash Flow Hedging Instruments									2,137	2,079	2,037
Changes in Unrealized Gains/(Losses) on Marketable Securities									2,294	(2,588)	315
Change in Funded Status of Pension, SERP and PBOP Benefit Plans									(2,869)	7,674	(30,330)
Other Comprehensive Income/(Loss), Net of Tax									1,562	7,165	(27,978)
Comprehensive Income									943,864	885,650	791,568
Eversource [Member]
Condensed Statement of Income Captions [Line Items]
Net Income									942,302	878,485	819,546
Other Comprehensive Income/(Loss), Net of Tax:
Other Comprehensive Income/(Loss), Net of Tax									1,562	7,165	(27,978)
Comprehensive Income									$ 943,864	$ 885,650	$ 791,568