Defined Contribution Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Contribution Plan, Cost Recognized	$ 25.7	$ 17.4	$ 16.1
The Connecticut Light And Power Company [Member]
Defined Contribution Plan, Cost Recognized	4.8	4.5	4.4
Western Massachusetts Electric Company [Member]
Defined Contribution Plan, Cost Recognized	0.9	0.9	0.9
Public Service Company Of New Hampshire [Member]
Defined Contribution Plan, Cost Recognized	3.3	3.1	2.8
NSTAR Electric Company [Member]
Defined Contribution Plan, Cost Recognized	$ 9.0	$ 8.7	$ 8.1