Fair Value Measurements (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (872.7)	$ (833.9)	$ (840.2)	$ (761.2)
Net Realized/Unrealized Gains Losses included in Net Income	0.5	0.7	0.8	0.5
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(13.9)	(23.7)	(45.2)	(96.0)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	(0.1)	0.1	(1.6)	(0.1)
Ending Balance	(886.2)	(856.8)	(886.2)	(856.8)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	0.2	0.5	0.6	(0.1)
The Connecticut Light and Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(842.6)	(790.5)	(806.1)	(715.8)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(13.9)	(23.7)	(45.1)	(95.4)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	(2.2)	(2.3)	(7.5)	(5.3)
Ending Balance	(858.7)	(816.5)	(858.7)	(816.5)
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	(843.9)	(792.9)	(808.0)	(720.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(13.0)	(23.1)	(43.2)	(91.8)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	(2.7)	(2.3)	(8.4)	(6.2)
Ending Balance	(859.6)	(818.3)	(859.6)	(818.3)
The Connecticut Light and Power Company [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	1.3	2.4	1.9	4.5
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.9)	(0.6)	(1.9)	(3.6)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	0.5	0	0.9	0.9
Ending Balance	0.9	1.8	0.9	1.8
Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(843.9)	(792.9)	(808.0)	(720.3)
Net Realized/Unrealized Gains Losses included in Net Income	0	0	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(13.0)	(23.1)	(43.2)	(91.8)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	(2.7)	(2.3)	(8.4)	(6.2)
Ending Balance	(859.6)	(818.3)	(859.6)	(818.3)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	0	0	0	0
Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(28.8)	(41.0)	(32.2)	(40.9)
Net Realized/Unrealized Gains Losses included in Net Income	0.5	0.7	0.8	0.5
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.9)	(0.6)	(2.0)	(4.2)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements	2.6	2.4	6.8	6.1
Ending Balance	(26.6)	(38.5)	(26.6)	(38.5)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	$ 0.2	$ 0.5	$ 0.6	$ (0.1)