Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (962.2)	$ (840.2)
Net Realized/Unrealized Gains Losses included in Net Income	8.0	0.3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	6.0	(31.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	14.5	(1.6)
Ending Balance	(901.5)	(872.7)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	4.4	0.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	32.2
Level 2 Unrealized Gains Netincome	4.6
Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(939.3)	(808.0)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	6.1	(30.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.0	(5.8)
Ending Balance	(912.2)	(843.9)
Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(22.9)	(32.2)
Net Realized/Unrealized Gains Losses included in Net Income	8.0	0.3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.1)	(1.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(6.5)	4.2
Ending Balance	10.7	(28.8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	32.2
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(931.6)	(806.1)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	10.9	(31.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.1	(5.4)
Ending Balance	(899.6)	(842.6)
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	(932.0)	(808.0)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	11.1	(30.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.0	(5.8)
Ending Balance	(899.9)	(843.9)
The Connecticut Light And Power Company [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	0.4	1.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(0.2)	(1.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0.1	0.4
Ending Balance	0.3	1.3
Western Massachusetts Electric Company [Member] | Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(7.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(5.0)
Ending Balance	$ (12.3)