MARKETABLE SECURITIES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	$ 430.2		$ 430.2		$ 450.8
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	56.9		56.9
Available For Sale Securities Debt Maturities After Ten Years Fair Value	121.4		121.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	68.0		68.0
Available For Sale Securities Debt Maturities Within One Year Fair Value	68.1		68.1
Available For Sale Securities Debt Maturities Fair Value	314.4		314.4
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	67.6		67.6
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	68.1		68.1
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	119.0		119.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	56.9		56.9
Available For Sale Securities Debt Maturities Amortized Cost	311.6		311.6
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	596.5		596.5		639.2
TradingSecuritiesEquity	14.2		14.2		85.1
IncreaseDecreaseInTradingSecurities	(0.5)	$ (1.9)	1.6	$ 1.9
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	0.5		0.5
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	14.2		14.2
Available For Sale Securities Debt Maturities Within One Year Fair Value	40.5		40.5
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	14.2		14.2
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	40.5		40.5
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	0.5		0.5
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	312.8		312.8		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	8.0		8.0		4.3
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	283.7		283.7		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	50.3		50.3		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	41.3		41.3		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	57.5		57.5		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	12.2		12.2		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	27.9		27.9		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	7.6		7.6		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	123.9		123.9		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	10.3		10.3		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	33.1		33.1		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	20.2		20.2		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	30.7		30.7		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	8.0		8.0		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	282.1		282.1		319.0
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	314.4		314.4		320.2
Available-for-sale Securities, Gross Unrealized Gains	3.6				7.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	(0.8)				(0.3)
Available For Sale Securities Amortized Cost	311.6		311.6		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	267.9		267.9		233.9
Available-for-sale Securities, Gross Unrealized Gains	53.9				73.3
AvailableForSaleSecuritiesGrossUnrealizedLoss	(6.0)				0.0
Available For Sale Securities Amortized Cost	$ 220.0		$ 220.0		$ 160.6