Defined Contribution Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Contribution Plan, Cost Recognized	$ 29.7	$ 37.0	$ 25.7
The Connecticut Light And Power Company [Member]
Defined Contribution Plan, Cost Recognized	5.0	5.1	4.8
NSTAR Electric Company [Member]
Defined Contribution Plan, Cost Recognized	6.3	8.5	9.0
Western Massachusetts Electric Company [Member]
Defined Contribution Plan, Cost Recognized	1.0	1.0	0.9
Public Service Company Of New Hampshire [Member]
Defined Contribution Plan, Cost Recognized	$ 3.2	$ 3.3	$ 3.3