PRESENTATION (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Reclassification of Transmission Expense	$ 42.2		$ 64.8
Reclassification of Energy Efficiency Expense	30.0		64.4
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 approximately $11.4 million, after-tax, for the first quarter of 2012.
The Connecticut Light And Power Company [Member]
Reclassification of Transmission Expense	23.2		39.8
Reclassification of Energy Efficiency Expense	21.3		44.7
Public Service Company Of New Hampshire [Member]
Reclassification of Transmission Expense	5.0		12.9
Reclassification of Energy Efficiency Expense	2.8		5.8
Western Massachusetts Electric Company [Member]
Reclassification of Transmission Expense	5.3		10.4
Reclassification of Energy Efficiency Expense	$ 5.0		$ 10.4