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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 85.1	$ 57.2
Fair Value, Option, Changes in Fair Value, Gain (Loss)	1.9	10.2	5.9
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	450.8	424.0
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	67.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	108.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	89.1
Available For Sale Securities Debt Maturities Within One Year Fair Value	55.0
Available For Sale Securities Debt Maturities Fair Value	320.2
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	88.8
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	55.0
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	103.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	66.0
Available For Sale Securities Debt Maturities Amortized Cost	313.0
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	639.2us-gaap_InvestmentsFairValueDisclosure	580.9us-gaap_InvestmentsFairValueDisclosure
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	0.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	2.9
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	25.8
Available For Sale Securities Debt Maturities Within One Year Fair Value	28.7
Available For Sale Securities Debt Maturities Fair Value	58.1
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	25.8
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	28.7
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	3.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	0.7
Available For Sale Securities Debt Maturities Amortized Cost	58.2
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	58.1	57.9
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	343.9	314.2
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	4.3	10.9
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	295.3	266.7
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	53.8	47.0
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	51.3	61.4
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	6.8
Corporate Bond Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	49.1	53.6
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	14.7	15.1
Asset Backed Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	54.1	30.4
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	14.5	9.0
Municipal Bonds Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	116.3	105.5
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	13.0	11.2
Other Debt Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	24.5	15.8
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	11.6	4.9
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	24.9	32.9
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	4.3	10.9
Mutual Funds [Member] \| Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	319.0	281.3
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	320.2	299.6
Available-for-sale Securities, Gross Unrealized Gains	7.5	2.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	0.3	2.1
Available For Sale Securities Amortized Cost	313.0	299.2
Debt Securities \| Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	58.1	57.9
Available-for-sale Securities, Gross Unrealized Gains	0	0
AvailableForSaleSecuritiesGrossUnrealizedLoss	0.1	0
Available For Sale Securities Amortized Cost	58.2	57.9
Equity Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	233.9	224.1
Available-for-sale Securities, Gross Unrealized Gains	73.3	60.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	0	0
Available For Sale Securities Amortized Cost	$ 160.6	$ 163.6