Basis of Presentation (Details)	6 Months Ended
	Jun. 30, 2016
Description of Consolidation of Yankee Companies	Eversource consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's, PSNH's and WMECO's combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Access Northeast is a natural gas pipeline and storage project (the "Project") being developed jointly by Eversource, Spectra Energy Corp and National Grid. Access Northeast will enhance the Algonquin and Maritimes & Northeast pipeline systems using existing routes. Eversource and Spectra Energy Corp each own a 40 percent interest in the Project, with the remaining 20 percent interest owned by National Grid. The total projected cost for both the pipeline and the LNG storage is expected to be approximately $3 billion, to be funded in proportion to ownership interest (approximately $1.2 billion by Eversource), with anticipated in-service dates commencing in November 2018. Eversource’s cumulative equity investment in the Project as of June 30, 2016 of $20.7 million is presented in Other Long-Term Assets.