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DERIVATIVE INSTURMENTS (Details) (USD $)
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current$ (110,653,000) $ (110,653,000) $ (71,501,000)
Derivative Liabilities, Noncurrent(909,252,000) (909,252,000) (909,668,000)
Derivative Assets Noncurrent91,430,000 91,430,000 123,242,000
Current Derivative Assets Consolidated14,200,000 14,200,000 17,300,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract     
Description of Derivative Activity VolumeAs of September 30, 2011 and December 31, 2010, NU had approximately 0.1 million and 0.3 million MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with contracted supply, both of which extend through 2013.    
Effect Of Fair Value Hedges On Results Of Operations [Abstract]     
Notional Amount Of Fair Value Hedge Instruments263,000,000 263,000,000  
Increase Decrease In Fair Value Of Interest Rate Fair Value Hedging Instruments(200,000)2,800,0001,100,00010,200,000 
Increase Decrease In Fair Value Of Hedged Item In Interest Rate Fair Value Hedge200,000(2,800,000)(1,100,000)(10,200,000) 
Interest Realized In Earnings On Fair Value Hedge2,500,0002,900,0007,900,0008,200,000 
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]     
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease(18,300,000) (25,100,000)  
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net(400,000)(100,000)(600,000)(300,000) 
Derivative Credit Risk Related Contingent Features Abstract     
Derivative Net Liability Position Aggregate Fair Value22,100,000 22,100,000 30,900,000
Collateral Already Posted Aggregate Fair Value0 0 500,000
Standby Letters Of Credit Posted6,000,000 6,000,000 24,000,000
Additional Collateral Aggregate Fair Value19,100,000 19,100,000 18,500,000
The Connecticut Light and Power Company [Member] | Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current(11,200,000) (11,200,000)  
Derivative Liabilities - Current    7,700,000
Derivative Assets Noncurrent(77,500,000) (77,500,000) (80,000,000)
Netting And Collateral [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current(11,200,000) (11,200,000)  
Derivative Liabilities - Current200,000 200,000 8,200,000
Derivative Liabilities, Noncurrent200,000 200,000 200,000
Derivative Assets Noncurrent(77,500,000) (77,500,000) (80,000,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current200,000 200,000 500,000
Derivative Liabilities, Noncurrent200,000 200,000 200,000
Derivative Assets Noncurrent0 0  
The Connecticut Light and Power Company [Member] | Commodity Contracts Required By Regulation [Member]
     
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract     
Description of Derivative Activity Volume  CL&P commodity and capacity contracts required by regulation: CL&P has capacity related contracts with generation facilities. These contracts and similar UI contracts, have an expected capacity of 787 MW. CL&P has a sharing agreement with UI, with 80 percent allocated to CL&P and 20 percent allocated to UI. The capacity contracts have terms up to 15 years and obligate the utilities to make or receive payments on a monthly basis to or from the generation facilities based on the difference between a set capacity price and the forward capacity market price received in the ISO-NE capacity markets. The largest of these generation facilities achieved commercial operation in July 2011. In addition, CL&P has a contract to purchase 0.1 million MWh of energy per year through 2020.  
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract     
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability(47,300,000)(49,800,000)(90,400,000)(141,600,000) 
The Connecticut Light and Power Company [Member] | Commodity Contracts Required By Regulation [Member] | Fair Value Inputs Level 3 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current17,200,000 17,200,000 5,800,000
Derivative Liabilities - Current(93,900,000) (93,900,000) (54,300,000)
Derivative Liabilities, Noncurrent(891,700,000) (891,700,000) (883,100,000)
Derivative Assets Noncurrent165,600,000 165,600,000 195,900,000
Commodity Contracts Required By Regulation [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current17,200,000 17,200,000 5,800,000
Derivative Liabilities - Current(93,900,000) (93,900,000) (54,300,000)
Derivative Liabilities, Noncurrent(891,700,000) (891,700,000) (883,100,000)
Derivative Assets Noncurrent165,600,000 165,600,000 195,900,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract     
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability(47,300,000)(49,800,000)(90,400,000)(141,600,000) 
Designated As Hedging Instrument [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current5,000,000 5,000,000 7,700,000
Derivative Assets Noncurrent    4,100,000
Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current5,000,000 5,000,000 7,700,000
Derivative Assets Noncurrent    4,100,000
The Connecticut Light and Power Company [Member] | Commodity [Member]
     
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract     
Description of Derivative Activity Volume  Commodity supply and price risk management: As of September 30, 2011 and December 31, 2010, CL&P had 0.5 million and 1.8 million MWh, respectively, remaining under FTRs that extend through December 2011 and require monthly payments or receipts.  
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract     
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability(600,000)(800,000)(2,500,000)(4,400,000) 
The Connecticut Light and Power Company [Member] | Commodity [Member] | Fair Value Inputs Level 3 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current500,000 500,000 2,100,000
Derivative Liabilities - Current(100,000) (100,000) (200,000)
Public Service Company of New Hampshire [Member] | Commodity [Member]
     
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract     
Description of Derivative Activity Volume  PSNH has electricity procurement contracts with delivery dates through 2011 to purchase an aggregate amount of 0.1 million and 0.4 million MWh of power as of September 30, 2011 and December 31, 2010, respectively. In addition, PSNH has 0.1 million and 0.3 million MWh remaining under FTRs as of September 30, 2011 and December 31, 2010, respectively, that extend through December 2011 and require monthly payments or receipts.  
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract     
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability200,000(2,100,000)200,000(17,800,000) 
Public Service Company of New Hampshire [Member] | Commodity [Member] | Fair Value Inputs Level 2 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current(3,300,000) (3,300,000) (12,800,000)
Commodity [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current3,200,000 3,200,000 3,800,000
Derivative Liabilities - Current(17,000,000) (17,000,000) (25,400,000)
Derivative Liabilities, Noncurrent(17,800,000) (17,800,000) (26,800,000)
Derivative Assets Noncurrent3,300,000 3,300,000 3,200,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract     
Derivative Instruments Gain Loss Recognized In Income Net300,0001,200,0001,000,0001,700,000 
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability(400,000)(2,900,000)(2,400,000)(22,700,000) 
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current2,700,000 2,700,000 1,700,000
Derivative Liabilities - Current(13,600,000) (13,600,000) (12,400,000)
Derivative Liabilities, Noncurrent(17,800,000) (17,800,000) (26,800,000)
Derivative Assets Noncurrent3,300,000 3,300,000 3,200,000
Public Service Company of New Hampshire [Member] | Fair Value Inputs Level 2 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current(3,300,000) (3,300,000) (12,800,000)
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current5,000,000 5,000,000 7,700,000
Derivative Assets Noncurrent    4,100,000
The Connecticut Light and Power Company [Member] | Fair Value Inputs Level 3 [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current6,500,000 6,500,000 7,900,000
Derivative Liabilities - Current(94,000,000) (94,000,000) (46,800,000)
Derivative Liabilities, Noncurrent(891,700,000) (891,700,000) (883,100,000)
Derivative Assets Noncurrent88,100,000 88,100,000 115,900,000
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Assets - Current2,700,000 2,700,000 1,700,000
Derivative Liabilities - Current(13,400,000) (13,400,000) (11,900,000)
Derivative Liabilities, Noncurrent(17,600,000) (17,600,000)  
Derivative Assets Noncurrent3,300,000 3,300,000 3,200,000
The Connecticut Light and Power Company [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current(93,987,000) (93,987,000) (46,781,000)
Derivative Liabilities, Noncurrent(891,709,000) (891,709,000) (883,091,000)
Derivative Assets Noncurrent88,099,000 88,099,000 115,870,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]     
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease0 0  
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net(200,000)(200,000)(600,000)(600,000) 
Public Service Company of New Hampshire [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities - Current(3,330,000) (3,330,000) (12,834,000)
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]     
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease(12,900,000) (18,200,000)  
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net(200,000)0(300,000)(100,000) 
Description Of Cash Flow Hedge Activity  PSNH entered into three forward starting swaps to fix the U.S. dollar LIBOR swap rate of 3.73 percent on $80 million of a planned $160 million long-term debt issuance, 2.79 percent on the remaining $80 million of the planned $160 million long-term debt issuance and 3.60 percent on $120 million of planned refinancing of PCRBs. In May 2011, PSNH settled the swap associated with the $120 million refinancing of PCRBs and a $2.9 million pre-tax reduction in AOCI will be amortized over the life of the debt. In September 2011, PSNH settled the two remaining swaps associated with the $160 million long-term debt issuance and a $15.3 million pre-tax reduction in AOCI will be amortized over the life of the debt.  
Derivative Credit Risk Related Contingent Features Abstract     
Derivative Net Liability Position Aggregate Fair Value3,300,000 3,300,000 12,800,000
Collateral Already Posted Aggregate Fair Value0 0 0
Standby Letters Of Credit Posted6,000,000 6,000,000 24,000,000
Additional Collateral Aggregate Fair Value500,000 500,000 0
Western Massachusetts Electric Company [Member]
     
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]     
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease(5,400,000) (6,900,000)  
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net000100,000 
Description Of Cash Flow Hedge Activity  WMECO entered into a forward starting swap to fix the U.S. dollar LIBOR swap rate of 3.75 percent associated with $50 million of a planned $100 million long-term debt issuance. In September 2011, WMECO settled the swap and a $6.9 million pre-tax reduction in AOCI will be amortized over the life of the debt.  
Subsidiaries [Member]
     
Derivative Instrument Detail Abstract     
Derivative Liabilities, Noncurrent    $ (26,600,000)