PRESENTATION (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Reclassification of Transmission Expense	$ 6.3		$ 71.1
Reclassification of Energy Efficiency Expense	35.3		99.7
Description of change in estimate		Effective January 1, 2012, NSTAR Electric increased its estimates with respect to the allowance for doubtful accounts, incurred but not reported claims on medical benefits, general and workers' compensation liabilities and various compensation accruals. The total aggregate impact of these increases in estimates to NSTAR Electric's accompanying unaudited condensed consolidated statements was $11.4 million, after-tax, for the first quarter and first nine months of 2012.
Description of Consolidation of Yankee Cos		On April 10, 2012, upon consummation of the merger with NSTAR, NSTAR Electric's ownership in CYAPC and YAEC combined with CL&P's, PSNH's and WMECO's respective ownership interests in CYAPC and YAEC totaled greater than 50 percent, requiring NU to consolidate CYAPC and YAEC. The investment in CYAPC and YAEC had previously been accounted for under the equity method by NU. The consolidation of CYAPC and YAEC results in NU recording nuclear decommissioning trust marketable securities of $324.9 million, regulatory assets of $212.9 million, long-term debt associated with the long-term fuel disposal liabilities of $179.5 million, net accumulated deferred income tax liability of $58.8 million and asset retirement obligations related to decommissioning activity of $292.9 million as of September 30, 2012. At the NU consolidated level, intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and CYAPC and YAEC 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.
The Connecticut Light And Power Company [Member]
Reclassification of Transmission Expense	8.7		48.5
Reclassification of Energy Efficiency Expense	24.9		69.6
Public Service Company Of New Hampshire [Member]
Reclassification of Transmission Expense	(0.3)		12.7
Reclassification of Energy Efficiency Expense	3.7		9.6
Western Massachusetts Electric Company [Member]
Reclassification of Transmission Expense	2.1		12.4
Reclassification of Energy Efficiency Expense	$ 6.1		$ 16.5