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MARKETABLE SECURITIES (Details) (USD $)
In Millions
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Schedule Of Available For Sale Securities Line Items     
Fair Value Option Changes In Fair Value Gain Loss$ 7.1$ 5.7$ 4.9$ 3.2 
Investments, Debt and Equity Securities [Abstract]     
Available For Sale Securities Fair Value Disclosure89.4 89.4 87.3
Available-for-sale Securities, Gross Unrealized Gains1.9 1.9 1.3
Available-for-sale Securities, Gross Unrealized Losses(0.3) (0.3) (0.3)
Available For Sale Securities Amortized Cost87.8 87.8 86.3
Available For Sale Securities Debt Maturities Fair Value Abstract     
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value11.1 11.1  
Available For Sale Securities Debt Maturities After Ten Years Fair Value24.9 24.9  
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value16.2 16.2  
Available For Sale Securities Debt Maturities Within One Year Fair Value37.2 37.2  
Available For Sale Securities Debt Maturities Fair Value89.4 89.4  
Available For Sale Securities Debt Maturities Amortized Cost Abstract     
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost16.1 16.1  
Available For Sale Securities Debt Maturities Within One Year Amortized Cost37.2 37.2  
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost23.8 23.8  
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost10.7 10.7  
Available For Sale Securities Debt Maturities Amortized Cost87.8 87.8  
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure126.7 126.7 129.5
Western Massachusetts Electric Company [Member] | US Government Debt Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure    6.0
US Government Debt Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure2.9 2.9 17.8
Western Massachusetts Electric Company [Member] | Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure8.5 8.5 15.6
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure17.1 17.1 22.5
Western Massachusetts Electric Company [Member] | Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure8.5 8.5 4.7
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure27.4 27.4 11.6
Western Massachusetts Electric Company [Member] | Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure15.7 15.7 15.4
Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure16.4 16.4 16.1
Western Massachusetts Electric Company [Member] | Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure24.0 24.0 15.1
Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure24.3 24.3 17.5
Western Massachusetts Electric Company [Member] | Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure0.4 0.4 0.3
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure1.3 1.3 1.8
Mutual Funds [Member] | Fair Value Inputs Level 1 Member
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure37.3 37.3 42.2
Western Massachusetts Electric Company [Member] | Fair Value Inputs Level 1 Member
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure0.4 0.4 0.3
Fair Value Inputs Level 1 Member
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure38.6 38.6 44.0
Western Massachusetts Electric Company [Member] | Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure56.7 56.7 56.8
Fair Value Inputs Level 2 [Member]
     
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure88.1 88.1 85.5
Western Massachusetts Electric Company [Member]
     
Investments, Debt and Equity Securities [Abstract]     
Available For Sale Securities Fair Value Disclosure57.1 57.1 57.1
Available-for-sale Securities, Gross Unrealized Losses(0.2) (0.2) (0.1)
Available For Sale Securities Amortized Cost57.3 57.3 57.2
Available For Sale Securities Debt Maturities Fair Value Abstract     
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value6.7 6.7  
Available For Sale Securities Debt Maturities After Ten Years Fair Value4.9 4.9  
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value10.6 10.6  
Available For Sale Securities Debt Maturities Within One Year Fair Value34.9 34.9  
Available For Sale Securities Debt Maturities Fair Value57.1 57.1  
Available For Sale Securities Debt Maturities Amortized Cost Abstract     
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost10.7 10.7  
Available For Sale Securities Debt Maturities Within One Year Amortized Cost34.9 34.9  
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost5.0 5.0  
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost6.7 6.7  
Available For Sale Securities Debt Maturities Amortized Cost57.3 57.3  
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract     
Investments Fair Value Disclosure$ 57.1 $ 57.1 $ 57.1