Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 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	0	5.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	49.2	26.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.7	13.6
Ending Balance	(564.3)	(833.1)
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	52.0	24.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.6	22.3
Ending Balance	(557.0)	(819.6)
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	(0.1)	0.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0.1	0.6
Ending Balance	$ (7.3)	$ (13.6)