Fair Value Measurements (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (886.2)	$ (856.8)	$ (840.2)	$ (761.2)
Net Realized/Unrealized Gains Losses included in Net Income	0.3	1.2	1.0	1.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(47.9)	(50.7)	(93.0)	(146.7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	17.7	(3.5)	16.1	(3.6)
Ending Balance	(916.1)	(909.8)	(916.1)	(909.8)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	0.1	1.0	0.7	0.9
The Connecticut Light and Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(858.7)	(816.5)	(806.1)	(715.8)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(47.9)	(50.6)	(92.9)	(146.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	15.5	(5.9)	7.9	(11.2)
Ending Balance	(891.1)	(873.0)	(891.1)	(873.0)
The Connecticut Light and Power Company [Member] | Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(859.6)	(818.3)	(808.0)	(720.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(47.3)	(49.8)	(90.4)	(141.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	15.4	(5.7)	6.9	(11.9)
Ending Balance	(891.5)	(873.8)	(891.5)	(873.8)
The Connecticut Light and Power Company [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	0.9	1.8	1.9	4.5
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.6)	(0.8)	(2.5)	(4.4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0.1	(0.2)	1.0	0.7
Ending Balance	0.4	0.8	0.4	0.8
Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(859.6)	(818.3)	(808.0)	(720.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(47.3)	(49.8)	(90.4)	(141.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	15.4	(5.7)	6.9	(11.9)
Ending Balance	(891.5)	(873.8)	(891.5)	(873.8)
Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(26.6)	(38.5)	(32.2)	(40.9)
Net Realized/Unrealized Gains Losses included in Net Income	0.3	1.2	1.0	1.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.6)	(0.9)	(2.6)	(5.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	2.3	2.2	9.2	8.3
Ending Balance	(24.6)	(36.0)	(24.6)	(36.0)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	$ 0.1	$ 1.0	$ 0.7	$ 0.9