Severance Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Notes To Consolidated Financial Statements [Abstract]
Description Of Postemployment Benefits	For the years ended December 31, 2015, 2014 and 2013, Eversource recorded severance benefit expense of $4.7 million, $15 million and $9.7 million, respectively, in connection with organizational and cost saving initiatives, and, in 2014, the partial outsourcing of information technology functions. As of December 31, 2015 and 2014, the severance accrual totaled $9.3 million and $10.4 million, respectively, and was included in Other Current Liabilities on the balance sheets.
Postemployment Benefits Period Expense	$ 4.7	$ 15.0	$ 9.7
Postemployment Benefits Liability Current	$ 9.3	$ 10.4

Description of benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement, except for: a) benefits provided through a pension or postretirement benefit plan, b) individual deferred compensation arrangements, c) termination benefits pertaining to exit or disposal activities, and d) stock compensation plans. Benefits may be provided in cash or in kind and may be paid as a result of disability, layoff, death, or other event.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 712
-SubTopic 10
-Section 15
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6410066&loc=d3e79194-111664

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Nonretirement Postemployment Benefits
-URI http://asc.fasb.org/extlink&oid=6518940

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