Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Contribution Plan, Cost Recognized	$ 30.4	$ 29.7	$ 37.0
The Connecticut Light And Power Company [Member]
Defined Contribution Plan, Cost Recognized	4.8	5.0	5.1
NSTAR Electric Company [Member]
Defined Contribution Plan, Cost Recognized	6.3	6.3	8.5
Western Massachusetts Electric Company [Member]
Defined Contribution Plan, Cost Recognized	1.0	1.0	1.0
Public Service Company Of New Hampshire [Member]
Defined Contribution Plan, Cost Recognized	$ 3.4	$ 3.2	$ 3.3