MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 57.2	$ 47.0
Fair Value, Option, Changes in Fair Value, Gain (Loss)	10.2	5.9
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	424.0	340.4
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	59.3
Available For Sale Securities Debt Maturities After Ten Years Fair Value	105.3
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	62.7
Available For Sale Securities Debt Maturities Within One Year Fair Value	72.3
Available For Sale Securities Debt Maturities Fair Value	299.6
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	62.1
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	72.4
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	105.3
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	59.4
Available For Sale Securities Debt Maturities Amortized Cost	299.2
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	580.9	492.3
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	1.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	4.1
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	25.5
Available For Sale Securities Debt Maturities Within One Year Fair Value	26.6
Available For Sale Securities Debt Maturities Fair Value	57.9
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	25.6
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	26.5
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	4.1
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	1.7
Available For Sale Securities Debt Maturities Amortized Cost	57.9
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	57.9	57.7
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	314.2	252.7
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	10.9	5.2
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	266.7	239.6
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	47.0	52.5
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	61.4	69.9
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	6.8	18.7
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	53.6	33.0
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	15.1	7.0
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	30.4	28.5
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	9.0	10.9
Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	105.5	93.8
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	11.2	11.6
Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	15.8	14.4
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	4.9	4.3
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	32.9	40.2
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	10.9	5.2
Mutual Funds [Member] | Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	281.3	212.5
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	299.6	279.8
Available-for-sale Securities, Gross Unrealized Gains	2.5	13.3
AvailableForSaleSecuritiesGrossUnrealizedLoss	2.1	0.1
Available For Sale Securities Amortized Cost	299.2	266.6
Debt Securities | Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	57.9	(57.7)
Available-for-sale Securities, Gross Unrealized Gains	0	0.1
AvailableForSaleSecuritiesGrossUnrealizedLoss	0	0.1
Available For Sale Securities Amortized Cost	57.9	57.7
Equity Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	224.1	165.5
Available-for-sale Securities, Gross Unrealized Gains	60.5	20.0
AvailableForSaleSecuritiesGrossUnrealizedLoss	0	0
Available For Sale Securities Amortized Cost	$ 163.6	$ 145.5