MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended																																							9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Fair Value Inputs Level 1 Member	Dec. 31, 2013 Fair Value Inputs Level 1 Member	Sep. 30, 2014 Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 US Government Debt Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 US Government Debt Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 US Government Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 US Government Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Asset Backed Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Asset Backed Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 Asset Backed Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Asset Backed Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 Municipal Bonds Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Municipal Bonds Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Other Debt Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Other Debt Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2014 Other Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Other Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member	Dec. 31, 2013 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member	Sep. 30, 2014 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Dec. 31, 2013 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Sep. 30, 2014 Debt Securities	Dec. 31, 2013 Debt Securities	Sep. 30, 2014 Debt Securities Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Debt Securities Western Massachusetts Electric Company [Member]	Sep. 30, 2014 Equity Securities	Dec. 31, 2013 Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 58.2		$ 58.2		$ 57.2
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure																																										318.2	299.6	58.1	57.9	232.0	224.1
Available-for-sale Securities, Gross Unrealized Gains																																										7.8	2.5	0.1	0	71.1	60.5
AvailableForSaleSecuritiesGrossUnrealizedLoss																																										(0.2)	(2.1)	(0.1)	0
Available For Sale Securities Amortized Cost																																										310.6	299.2	58.1	57.9	160.9	163.6
Marketable Securities, Restricted	447.7		447.7		424.0
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	67.5		67.5			1.4
Available For Sale Securities Debt Maturities After Ten Years Fair Value	111.7		111.7			4.1
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	91.4		91.4			30.5
Available For Sale Securities Debt Maturities Within One Year Fair Value	47.6		47.6			22.1
Available For Sale Securities Debt Maturities Fair Value	318.2		318.2			58.1
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	91.0		91.0			30.7
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	47.6		47.6			21.9
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	106.4		106.4			4.1
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	65.6		65.6			1.4
Available For Sale Securities Debt Maturities Amortized Cost	310.6		310.6			58.1
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	632.8		632.8		580.9	58.1	57.9	341.7	314.2	4.4	10.9	291.1	266.7	53.7	47.0	56.5	61.4	0	6.8	61.2	53.6	14.1	15.1	37.9	30.4	14.3	9.0	112.5	105.5	13.9	11.2	23.0	15.8	11.4	4.9	27.1	32.9	4.4	10.9	314.6	281.3
TradingSecuritiesEquity	24.4		24.4
IncreaseDecreaseInTradingSecurities	$ (1.9)	$ 3.0	$ 1.9	$ 7.3