MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended																																							6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Western Massachusetts Electric Company [Member]	Jun. 30, 2014 Fair Value Inputs Level 1 Member	Dec. 31, 2013 Fair Value Inputs Level 1 Member	Jun. 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]	Jun. 30, 2014 Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Fair Value Inputs Level 2 [Member]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 30, 2014 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2013 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Jun. 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]	Jun. 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]	Jun. 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]	Jun. 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	Jun. 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]	Jun. 30, 2014 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Dec. 31, 2013 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Jun. 30, 2014 Debt Securities	Dec. 31, 2013 Debt Securities	Jun. 30, 2014 Debt Securities Western Massachusetts Electric Company [Member]	Dec. 31, 2013 Debt Securities Western Massachusetts Electric Company [Member]	Jun. 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	$ 59.6		$ 59.6		$ 57.2
Fair Value, Option, Changes in Fair Value, Gain (Loss)	2.2	0.1	2.4	4.3
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure																																										315.3	299.6	58.0	57.9	230.2	224.1
Available-for-sale Securities, Gross Unrealized Gains																																										7.3	2.5	0	0	66.7	60.5
AvailableForSaleSecuritiesGrossUnrealizedLoss																																										(0.3)	(2.1)	0	0
Available For Sale Securities Amortized Cost																																										308.3	299.2	58.0	57.9	163.5	163.6
Marketable Securities, Restricted	444.3		444.3		424.0
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	57.7		57.7			2.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	107.2		107.2			4.1
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	74.6		74.6			32.0
Available For Sale Securities Debt Maturities Within One Year Fair Value	75.8		75.8			19.2
Available For Sale Securities Debt Maturities Fair Value	315.3		315.3			58.0
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	73.9		73.9			31.9
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	75.9		75.9			19.2
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	102.4		102.4			4.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	56.1		56.1			2.7
Available For Sale Securities Debt Maturities Amortized Cost	308.3		308.3			58.0
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	630.0		630.0		580.9	58.0	57.9	357.0	314.2	2.0	10.9	273.0	266.7	56.0	47.0	40.6	61.4	0	6.8	61.8	53.6	15.8	15.1	36.5	30.4	15.2	9.0	109.5	105.5	11.7	11.2	24.6	15.8	13.3	4.9	42.3	32.9	2.0	10.9	314.7	281.3
TradingSecuritiesEquity	24.9		24.9
IncreaseDecreaseInTradingSecurities	$ 0.9		$ 1.4