Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (635.2)	$ (878.6)
Net Realized/Unrealized Gains Losses included in Net Income		10.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	141.3	158.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	78.5	74.2
Ending Balance	(415.4)	(635.2)
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(630.6)	(866.2)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	139.7	148.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	80.0	86.7
Ending Balance	(410.9)	(630.6)
NSTAR Electric Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(7.3)	(14.9)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	4.3	3.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(1.5)	4.1
Ending Balance	$ (4.5)	$ (7.3)