Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Site Contingency [Line Items]
Environmental Site Quantity	65	68
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 35.4	$ 39.4
Accrual For Environmental Loss Contingencies Provision For New Losses	12.7	3.5
Accrual for Environmental Loss Contingencies, Payments	(4.8)	(7.5)
Ending Balance	43.3	35.4
Site Contingency [Abstract]
Information on range of possible loss and superfunds	CERCLA: Of the total environmental sites, 9 sites (1 for CL&P, 3 for NSTAR Electric and 3 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, 2014, a liability of $0.7 million accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites. Included in the NU 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. The reserve balance related to these former MGP sites was $38.8 million and $31.4 million as of December 31, 2014 and 2013, respectively, and relates primarily to the natural gas business segment. The increase in the reserve balance for the MGP sites was due to the completion of the site assessment at three sites. The assessments provided new information related to the extent and nature of the contamination and the costs of required remediation. As of December 31, 2014, for 5 environmental sites (1 for CL&P, and 1 for WMECO) that are included in the Company's reserve for environmental costs, the information known and nature of the remediation options at those sites allow for the Company to estimate the range of losses for environmental costs. As of December 31, 2014, $17.7 million ($1 million for CL&P and $0.3 million for WMECO) had been accrued as a liability for these sites, which represent management's best estimates of the liabilities for environmental costs. These amounts are the best estimates with estimated ranges of additional losses from zero to $24 million. As of December 31, 2014, for 15 environmental sites (3 for CL&P, 3 for NSTAR Electric and 2 for PSNH) 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, 2014, $13.4 million ($1.4 million for CL&P, $0.2 million for PSNH) had been accrued as a liability for these sites. As of December 31, 2014, for the remaining 45 environmental sites (12 for CL&P, 10 for NSTAR Electric, 11 for PSNH, and 3 for WMECO) that are included in the Company’s reserve for environmental costs, the $12.2 million accrual ($1.4 million for CL&P, $1.1 million for NSTAR Electric, $5 million for PSNH, and $0.2 million for WMECO) represents management’s best estimate of the liability and no additional loss is anticipated.
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual For Environmental Loss Contingencies Component Amount	38.8	31.4
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	18
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.4	3.7
Accrual For Environmental Loss Contingencies Provision For New Losses	1.0	0.2
Accrual for Environmental Loss Contingencies, Payments	(0.6)	(0.5)
Ending Balance	3.8	3.4
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	12
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.2	1.7
Accrual For Environmental Loss Contingencies Provision For New Losses	0	0.2
Accrual for Environmental Loss Contingencies, Payments	(0.1)	(0.7)
Ending Balance	1.1	1.2
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	15
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	5.4	4.9
Accrual For Environmental Loss Contingencies Provision For New Losses	0.1	1.0
Accrual for Environmental Loss Contingencies, Payments	(0.3)	(0.5)
Ending Balance	5.2	5.4
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	5
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	0.4	0.6
Accrual For Environmental Loss Contingencies Provision For New Losses	0.2	0
Accrual for Environmental Loss Contingencies, Payments	(0.1)	(0.2)
Ending Balance	$ 0.5	$ 0.4