MARKETABLE SECURITIES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	$ 450.0		$ 450.0		$ 450.8
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	61.2		61.2
Available For Sale Securities Debt Maturities After Ten Years Fair Value	121.5		121.5
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	70.8		70.8
Available For Sale Securities Debt Maturities Within One Year Fair Value	66.1		66.1
Available For Sale Securities Debt Maturities Fair Value	319.6		319.6
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	70.4		70.4
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	66.1		66.1
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	118.7		118.7
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	60.1		60.1
Available For Sale Securities Debt Maturities Amortized Cost	315.3		315.3
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	622.7		622.7		639.2
TradingSecuritiesEquity	14.9		14.9		85.1
IncreaseDecreaseInTradingSecurities	0.5	$ 3.1	2.1	$ 3.8
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	0.6		0.6
Available For Sale Securities Debt Maturities After Ten Years Fair Value	3.1		3.1
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	16.2		16.2
Available For Sale Securities Debt Maturities Within One Year Fair Value	38.4		38.4
Available For Sale Securities Debt Maturities Fair Value	58.3		58.3
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	16.2		16.2
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	38.4		38.4
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	3.1		3.1
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	0.6		0.6
Available For Sale Securities Debt Maturities Amortized Cost	58.3		58.3
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	58.3		58.3		58.1
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	329.5		329.5		343.9
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	2.1		2.1		4.3
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	293.2		293.2		295.3
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	56.2		56.2		53.8
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	38.6		38.6		51.3
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	0.0		0.0		0.0
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	60.1		60.1		49.1
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	13.6		13.6		14.7
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	32.0		32.0		54.1
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	10.0		10.0		14.5
Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	126.9		126.9		116.3
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	10.8		10.8		13.0
Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	35.6		35.6		24.5
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	21.8		21.8		11.6
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	26.4		26.4		24.9
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	2.1		2.1		4.3
Mutual Funds [Member] | Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	303.1		303.1		319.0
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	319.6		319.6		320.2
Available-for-sale Securities, Gross Unrealized Gains	4.9				7.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	(0.6)				(0.3)
Available For Sale Securities Amortized Cost	315.3		315.3		313.0
Debt Securities | Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	58.3		58.3		58.1
Available-for-sale Securities, Gross Unrealized Gains	0.0				0.0
AvailableForSaleSecuritiesGrossUnrealizedLoss	0.0				(0.1)
Available For Sale Securities Amortized Cost	58.3		58.3		58.2
Equity Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	288.2		288.2		233.9
Available-for-sale Securities, Gross Unrealized Gains	72.9				73.3
AvailableForSaleSecuritiesGrossUnrealizedLoss	(1.2)				0.0
Available For Sale Securities Amortized Cost	$ 216.5		$ 216.5		$ 160.6