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MARKETABLE SECURITIES (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Schedule Of Available For Sale Securities [Line Items]    
Fair Value Of Securities Under Fair Value Option $ 47.0 $ 41.1
Fair Value, Option, Changes in Fair Value, Gain (Loss) 5.9 (1.1)
Investments, Debt and Equity Securities [Abstract]    
Marketable Securities, Restricted 340.4  
Available For Sale Securities Debt Maturities Fair Value Abstract    
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value 54.6  
Available For Sale Securities Debt Maturities After Ten Years Fair Value 99.9  
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value 58.7  
Available For Sale Securities Debt Maturities Within One Year Fair Value 66.6  
Available For Sale Securities Debt Maturities Fair Value 279.8  
Available For Sale Securities Debt Maturities Amortized Cost Abstract    
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost 57.3  
Available For Sale Securities Debt Maturities Within One Year Amortized Cost 66.6  
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost 91.5  
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost 51.2  
Available For Sale Securities Debt Maturities Amortized Cost 266.6  
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 492.3 131.3
Western Massachusetts Electric Company [Member]
   
Available For Sale Securities Debt Maturities Fair Value Abstract    
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value 6.1  
Available For Sale Securities Debt Maturities After Ten Years Fair Value 6.7  
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value 17.5  
Available For Sale Securities Debt Maturities Within One Year Fair Value 27.4  
Available For Sale Securities Debt Maturities Fair Value 57.7  
Available For Sale Securities Debt Maturities Amortized Cost Abstract    
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost 17.5  
Available For Sale Securities Debt Maturities Within One Year Amortized Cost 27.4  
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost 6.8  
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost 6.0  
Available For Sale Securities Debt Maturities Amortized Cost 57.7  
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 57.7 57.1
Fair Value Inputs Level 1 Member
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 252.7 42.9
Fair Value Inputs Level 1 Member | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 5.2 0.1
Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 239.6 88.4
Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 52.5 57.0
US Government Debt Securities Member | Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 69.9 11.1
US Government Debt Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 18.7 8.0
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 33.0 16.5
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 7.0 9.1
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 28.5 25.9
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 10.9 7.9
Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 93.8 16.1
Municipal Bonds Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 11.6 15.4
Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 14.4 18.8
Other Debt Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 4.3 16.6
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 40.2 1.8
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member | Western Massachusetts Electric Company [Member]
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 5.2 0.1
Mutual Funds [Member] | Fair Value Inputs Level 1 Member
   
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract    
Investments Fair Value Disclosure 212.5 41.1
Debt Securities
   
Investments, Debt and Equity Securities [Abstract]    
Available For Sale Securities Fair Value Disclosure 279.8 90.2
Available-for-sale Securities, Gross Unrealized Gains 13.3 2.0
Available-for-sale Securities, Gross Unrealized Losses (0.1) (0.2)
Available For Sale Securities Amortized Cost 266.6 88.4
Debt Securities | Western Massachusetts Electric Company [Member]
   
Investments, Debt and Equity Securities [Abstract]    
Available For Sale Securities Fair Value Disclosure 57.7 57.1
Available-for-sale Securities, Gross Unrealized Gains 0.1  
Available-for-sale Securities, Gross Unrealized Losses (0.1) (0.2)
Available For Sale Securities Amortized Cost 57.7 57.3
Equity Securities
   
Investments, Debt and Equity Securities [Abstract]    
Available For Sale Securities Fair Value Disclosure 165.5  
Available-for-sale Securities, Gross Unrealized Gains 20.0  
Available For Sale Securities Amortized Cost $ 145.5