Defined Contribution Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
K Vantage[ Member]
Defined Contribution Plan, Cost Recognized	$ 4.2	$ 3.4	$ 2.6
Savings Plan 401k [Member]
Defined Contribution Plan, Cost Recognized	13.2	12.7	12.2
The Connecticut Light And Power Company [Member] | K Vantage[ Member]
Defined Contribution Plan, Cost Recognized	0.5	0.4	0.2
The Connecticut Light And Power Company [Member] | Savings Plan 401k [Member]
Defined Contribution Plan, Cost Recognized	4.0	4.0	3.9
Western Massachusetts Electric Company [Member] | K Vantage[ Member]
Defined Contribution Plan, Cost Recognized	0.1	0.1	0
Western Massachusetts Electric Company [Member] | Savings Plan 401k [Member]
Defined Contribution Plan, Cost Recognized	0.8	0.8	0.7
Public Service Company Of New Hampshire [Member] | K Vantage[ Member]
Defined Contribution Plan, Cost Recognized	0.6	0.4	0.3
Public Service Company Of New Hampshire [Member] | Savings Plan 401k [Member]
Defined Contribution Plan, Cost Recognized	$ 2.5	$ 2.4	$ 2.3