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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Fair Value Inputs Level 1 Member	Dec. 31, 2012 Fair Value Inputs Level 1 Member	Sep. 30, 2013 Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 US Government Debt Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 US Government Debt Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 US Government Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 US Government Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Corporate Bond Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Asset Backed Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 Asset Backed Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 Asset Backed Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Asset Backed Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 Municipal Bonds Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 Municipal Bonds Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Municipal Bonds Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Other Debt Securities Member Fair Value Inputs Level 2 [Member]	Dec. 31, 2012 Other Debt Securities Member Fair Value Inputs Level 2 [Member]	Sep. 30, 2013 Other Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Other Debt Securities Member Fair Value Inputs Level 2 [Member] Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member	Dec. 31, 2012 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member	Sep. 30, 2013 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Cash And Cash Equivalents Member Fair Value Inputs Level 1 Member Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Dec. 31, 2012 Mutual Funds [Member] Fair Value Inputs Level 1 Member	Sep. 30, 2013 Debt Securities	Dec. 31, 2012 Debt Securities	Sep. 30, 2013 Debt Securities Western Massachusetts Electric Company [Member]	Dec. 31, 2012 Debt Securities Western Massachusetts Electric Company [Member]	Sep. 30, 2013 Equity Securities	Dec. 31, 2012 Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 54.3		$ 54.3		$ 47.0
Fair Value, Option, Changes in Fair Value, Gain (Loss)	3.0	1.9	7.3	4.6
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure																																										303.8	279.8	57.8	57.7	204.9	165.5
Available-for-sale Securities, Gross Unrealized Gains																																										1.5	13.3	0	0.1	40.9	20.0
AvailableForSaleSecuritiesGrossUnrealizedLoss																																										(3.8)	(0.1)	0	(0.1)
Available For Sale Securities Amortized Cost																																										306.1	266.6	57.8	57.7	164.0	145.5
Marketable Securities, Restricted	403.1		403.1		340.4
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	57.3		57.3			2.5
Available For Sale Securities Debt Maturities After Ten Years Fair Value	104.6		104.6			4.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	76.6		76.6			26.3
Available For Sale Securities Debt Maturities Within One Year Fair Value	65.3		65.3			24.6
Available For Sale Securities Debt Maturities Fair Value	303.8		303.8			57.8
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	76.0		76.0			26.4
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	67.1		67.1			24.4
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	104.6		104.6			4.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	58.4		58.4			2.5
Available For Sale Securities Debt Maturities Amortized Cost	306.1		306.1			57.8
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	$ 563.0		$ 563.0		$ 492.3	$ 57.8	$ 57.7	$ 299.2	$ 252.7	$ 2.8	$ 5.2	$ 263.8	$ 239.6	$ 55.0	$ 52.5	$ 74.9	$ 69.9	$ 16.6	$ 18.7	$ 48.9	$ 33.0	$ 15.1	$ 7.0	$ 30.4	$ 28.5	$ 9.6	$ 10.9	$ 93.7	$ 93.8	$ 8.8	$ 11.6	$ 15.9	$ 14.4	$ 4.9	$ 4.3	$ 40.0	$ 40.2	$ 2.8	$ 5.2	$ 259.2	$ 212.5