Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Site Contingency [Line Items]
Environmental Site Quantity	77	59
Range Of Potential Loss On Enivornmental Sites	As of December 31, 2012, for 8 environmental sites (3 for CL&P, 2 for PSNH, 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, 2012, $2.6 million ($0.6 million for CL&P and $0.7 million for PSNH) 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 million to $15.2 million (zero to $1.3 million for CL&P, zero to $4.1 million for PSNH, and zero to $8.6 million for WMECO). As of December 31, 2012, for 23 environmental sites (5 for CL&P,1 for NSTAR Electric, 4 for PSNH, and 3 for WMECO) 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, 2012, $17 million ($1.7 million for CL&P, $0.2 million for PSNH, and $0.5 million for WMECO) had been accrued as a liability for these sites. As of December 31, 2012, for the remaining 46 environmental sites (11 for CL&P, 15 for NSTAR Electric, 10 for PSNH, and 2 for WMECO) included in the Company’s reserve for environmental costs, the $19.8 million accrual ($1.4 million for CL&P, $1.7 million for NSTAR Electric, $4 million for PSNH, and $0.1 million for WMECO) represents management’s best estimate of the liability and no additional loss is anticipated.
Superfund Site Information	CERCLA: The federal Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA) and its amendments or state equivalents impose joint and several strict liabilities, regardless of fault, upon generators of hazardous substances resulting in removal and remediation costs and environmental damages.  Liabilities under these laws can be material and in some instances may be imposed without regard to fault or for past acts that may have been lawful at the time they occurred.  Of the total sites included in the remediation and long-term monitoring phase, 10 sites (2 for CL&P, 4 for NSTAR Electric, 4 for PSNH and 1 for WMECO) are superfund sites under CERCLA 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, 2012, a liability of $1 million ($0.4 million for CL&P, $0.1 million for NSTAR Electric and $0.4 million for PSNH) accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites.
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 31.7	$ 37.1
Environmental Liability assumed in merger	11.8
Accrual For Environmental Loss Contingencies Provision For New Losses	4.7	1.6
Accrual for Environmental Loss Contingencies, Payments	(8.8)	(7.0)
Ending Balance	39.4	31.7
Accrual For Environmental Loss Contingencies Component Amount	2.1
Site Contingency [Abstract]
Cumulative Payments Environmental Loss Contingencies	17.4
Cumulative Environmental Remediation Expense	19.5
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	19	18
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	2.9	2.8
Accrual For Environmental Loss Contingencies Provision For New Losses	1.3	0.4
Accrual for Environmental Loss Contingencies, Payments	(0.5)	(0.3)
Ending Balance	3.7	2.9
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	18
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	6.6	9.1
Accrual For Environmental Loss Contingencies Provision For New Losses	0.2	0.1
Accrual for Environmental Loss Contingencies, Payments	(1.9)	(2.6)
Ending Balance	4.9	6.6
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	6	10
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	0.3	0.3
Accrual For Environmental Loss Contingencies Provision For New Losses	0.5	0.1
Accrual for Environmental Loss Contingencies, Payments	(0.2)	(0.1)
Ending Balance	0.6	0.3
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	13
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.3	0.9
Accrual For Environmental Loss Contingencies Provision For New Losses	0.7	0.4
Accrual for Environmental Loss Contingencies, Payments	(0.3)	0
Ending Balance	$ 1.7	$ 1.3