Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reclassification of Transmission Expense		$ 77.2	$ 48.9
Reclassification of Energy Expense		131.4	124.0
Description of Consolidation of Yankee Companies	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 from April 10, 2012 and forward. 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 $340.4 million, regulatory assets of $214 million, long-term debt associated with the long-term spent nuclear fuel disposal liabilities of $179.3 million, net accumulated deferred income tax liability of $56.4 million and asset retirement obligations related to decommissioning activity of $311.4 million as of December 31, 2012. At the NU consolidated level, 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. See Note 1J, "Summary of Significant Accounting Policies – Equity Method Investments," for further information.
Immaterial Error Correction	The NSTAR Electric consolidated statements of cash flows were 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 $1.7 million and $24.1 million and a corresponding increase to financing cash outflows from Retirements ofRate Reduction Bonds for the years ended December 31, 2011 and 2010, respectively. These revisions had no impact on NSTAR Electric’s results of operations or cash balance and are not deemed material, individually orin the aggregate, to the previously issued consolidated financial statements.
The Connecticut Light And Power Company [Member]
Reclassification of Transmission Expense		52.6	39.4
Reclassification of Energy Expense		90.3	92.3
Public Service Company Of New Hampshire [Member]
Reclassification of Transmission Expense		19.1	26.4
Reclassification of Energy Expense		12.9	12.0
Western Massachusetts Electric Company [Member]
Reclassification of Transmission Expense		15.9	18.3
Reclassification of Energy Expense		$ 21.8	$ 16.3
NSTAR Electric Company [Member]
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 statements was a decrease to net income of $11.4 million, after-tax, for the year ended December 31, 2012.