Other Contingencies (Details) (FERC ROE Reserve - Incentive - First and Second Complaint [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Loss Contingencies Line Items
Loss Contingency Loss in Period	$ 20.0
The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	12.5
NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	2.4
Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	1.0
Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	$ 4.1