SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Statement of Income Captions [Line Items]
Net Income	$ 239,400	$ 262,200	$ 232,600	$ 261,300	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 995,515	$ 949,821	$ 886,004
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments									1,974	2,137	2,079
Changes in Unrealized Gains/(Losses) on Marketable Securities									(350)	2,294	(2,588)
Change in Funded Status of Pension, SERP and PBOP Benefit Plans									(2,745)	(2,869)	7,674
Other Comprehensive (Loss)/Income, Net of Tax									(1,121)	1,562	7,165
Comprehensive Income									986,875	943,864	885,650
Eversource
Condensed Statement of Income Captions [Line Items]
Net Income									987,996	942,302	878,485
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments									1,974	2,137	2,079
Changes in Unrealized Gains/(Losses) on Marketable Securities									(350)	2,294	(2,588)
Change in Funded Status of Pension, SERP and PBOP Benefit Plans									2,745	2,869	(7,674)
Other Comprehensive (Loss)/Income, Net of Tax									(1,121)	1,562	7,165
Comprehensive Income									$ 986,875	$ 943,864	$ 885,650