Environmental Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Site Contingency [Line Items]
Environmental Site Quantity	64	65
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 43.3	$ 35.4
Accrual For Environmental Loss Contingencies Provision For New Losses	13.5	12.7
Accrual for Environmental Loss Contingencies, Payments	(5.7)	(4.8)
Ending Balance	$ 51.1	43.3
Site Contingency [Abstract]
Information on range of possible loss and superfunds	CERCLA: Of the total environmental sites, nine sites (four for NSTAR Electric and three for PSNH) are superfund sites under the federal Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA) and its amendments or state equivalents for which the Company has been notified that it is a potentially responsible party but for which the site assessment and remediation are not being managed by the Company.  As of December 31, 2015, a liability of $0.8 million accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites. Included in the Eversource number of sites and reserve amounts above are former MGP sites that were operated several decades ago and manufactured gas from coal and other processes, which resulted in certain by-products remaining in the environment that may pose a potential risk to human health and the environment, for which Eversource may have potential liability. The reserve balances related to these former MGP sites were $45.5 million and $38.8 million as of December 31, 2015 and 2014, respectively, and related primarily to the natural gas business segment. As of December 31, 2015, for 9 environmental sites (3 for CL&P, 1 for WMECO) that are included in the Company's reserve for environmental costs, the information known and the nature of the remediation options allow for the Company to estimate the range of losses for environmental costs. As of December 31, 2015, $24.7 million (including $1.7 million for CL&P and $0.3 million for WMECO) had been accrued as a liability for these sites, which represents the low end of the range of the liabilities for environmental costs. Management believes that additional losses of up to approximately $33.9 million (approximately $1.4 million for CL&P, and $0.1 million for WMECO) may be incurred in remediating these sites. As of December 31, 2015, for 12 environmental sites (3 for CL&P and 2 for NSTAR Electric) that are included in the Company’s reserve for environmental costs, management cannot reasonably estimate the exposure to loss in excess of the reserve, or range of loss, as these sites are under investigation and/or there is significant uncertainty as to what remedial actions, if any, the Company may be required to undertake. As of December 31, 2015, $13.7 million (including $2 million for CL&P) had been accrued as a liability for these sites. As of December 31, 2015, for the remaining 43 environmental sites (including 8 for CL&P, 13 for NSTAR Electric, 12 for PSNH, and 3 for WMECO) that are included in the Company’s reserve for environmental costs, the $12.7 million accrual (including $0.9 million for CL&P, $2.4 million for NSTAR Electric, $4.5 million for PSNH, and $0.3 million for WMECO) represents management’s best estimate of the potential liability and no additional loss is anticipated at this time.
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual For Environmental Loss Contingencies Component Amount	$ 45.5	$ 38.8
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 3.8	$ 3.4
Accrual For Environmental Loss Contingencies Provision For New Losses	1.3	1.0
Accrual for Environmental Loss Contingencies, Payments	(0.5)	(0.6)
Ending Balance	$ 4.6	$ 3.8
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	15	13
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 1.1	$ 1.2
Accrual For Environmental Loss Contingencies Provision For New Losses	2.0	0.0
Accrual for Environmental Loss Contingencies, Payments	(0.7)	(0.1)
Ending Balance	$ 2.4	$ 1.1
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	12	13
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 5.2	$ 5.4
Accrual For Environmental Loss Contingencies Provision For New Losses	2.3	0.1
Accrual for Environmental Loss Contingencies, Payments	(3.0)	(0.3)
Ending Balance	$ 4.5	$ 5.2
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	4
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 0.5	$ 0.4
Accrual For Environmental Loss Contingencies Provision For New Losses	0.2	0.2
Accrual for Environmental Loss Contingencies, Payments	(0.1)	(0.1)
Ending Balance	$ 0.6	$ 0.5