Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (406.8)	$ (564.3)	$ (415.4)	$ (635.2)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(37.1)	111.8	(49.2)	161.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.5	21.6	42.2	43.0
Ending Balance	(422.4)	(430.9)	(422.4)	(430.9)
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(403.3)	(557.0)	(410.9)	(630.6)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(36.8)	112.2	(48.9)	164.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	19.9	20.2	39.6	41.5
Ending Balance	(420.2)	(424.6)	(420.2)	(424.6)
NSTAR Electric Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(3.5)	(7.3)	(4.5)	(7.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.3)	(0.4)	(0.3)	(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	1.6	1.4	2.6	1.5
Ending Balance	$ (2.2)	$ (6.3)	$ (2.2)	$ (6.3)