Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reclassification of Transmission Expense		$ (2.7)
Reclassification of Energy Expense		37.3
Description of Change in Estimate		Effective January 1, 2012, NSTAR Electric changed its estimates with respect to the allowance for doubtful accounts, incurred but not reported claims on medical benefits, general and workers' compensation liabilities and compensation accruals. The total aggregate impact of these changes in estimates on NSTAR Electric's accompanying consolidated statement of income was a decrease to net income of $11.4 million, after-tax, for the three months ended March 31, 2012.
Description of Consolidation of Yankee Companies	NU consolidates CYAPC and YAEC as 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. For CL&P, NSTAR Electric, PSNH and WMECO, the investment in CYAPC and YAEC continue to be accounted for under the equity method.
Immaterial Error Correction		The NSTAR Electric consolidated statement of cash flows was revised to correct an error in the presentation of cash deposits related to the RRBs. The impact of this revision was an increase in investing cash inflows from Other Investing Activities in an amount of $24.8 million and a corresponding increase to financing cash outflows from Retirements of Rate Reduction Bonds for the three months ended March 31, 2012. This revision had no impact on NSTAR Electric’s results of operations or cash balance and is not deemed material, individually or in the aggregate, to the previously issued consolidated financial statements.
The Connecticut Light And Power Company [Member]
Reclassification of Transmission Expense		(2.9)
Reclassification of Energy Expense		22.0
Public Service Company Of New Hampshire [Member]
Reclassification of Transmission Expense		7.9
Reclassification of Energy Expense		3.6
Western Massachusetts Electric Company [Member]
Reclassification of Transmission Expense		1.0
Reclassification of Energy Expense		$ 5.6