Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (840.2)	$ (761.2)
Net Realized/Unrealized Gains Losses included in Net Income	0.5	2.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(161.0)	(81.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	38.5	(0.5)
Ending Balance	(962.2)	(840.2)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	0.7	1.2
Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(808.0)	(720.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(158.1)	(74.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	26.8	(13.7)
Ending Balance	(939.3)	(808.0)
Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(32.2)	(40.9)
Net Realized/Unrealized Gains Losses included in Net Income	0.5	2.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(2.9)	(7.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	11.7	13.2
Ending Balance	(22.9)	(32.2)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings	0.7	1.2
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(806.1)	(715.8)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(153.6)	(80.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	28.1	(10.1)
Ending Balance	(931.6)	(806.1)
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	(808.0)	(720.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(150.8)	(74.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	26.8	(13.7)
Ending Balance	(932.0)	(808.0)
The Connecticut Light And Power Company [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	1.9	4.5
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(2.8)	(6.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	1.3	3.6
Ending Balance	0.4	1.9
Western Massachusetts Electric Company [Member] | Commodity Contracts Required By Regulation [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(7.3)
Ending Balance	$ (7.3)