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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any.

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-Name Statement of Financial Accounting Standard (FAS)
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This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, which are expected to mature after five years and through ten years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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-Name Statement of Financial Accounting Standard (FAS)
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This item represents the gross unrealized gains for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized gains are the excess of the fair value of the Available-for-sale Securities over their carrying value as of the reporting date. Such gross unrealized gains are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge. All or a portion of the unrealized holding gain of an available-for-sale security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge.

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This item represents, for each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings during the period and in which line in the income statement those gains and losses are reported. This item may also include amounts of gains and losses for other items measured at fair value, but for which the fair value option has not been elected (for instance, items required to be measured at fair value).

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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph a, f, g
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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MARKETABLE SECURITIES (Details) (USD $) In Millions	3 Months Ended		6 Months Ended		12 Months Ended
MARKETABLE SECURITIES (Details) (USD $) In Millions	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Schedule Of Available For Sale Securities Line Items
Fair Value Of Securities Under Fair Value Option	$ 44.4		$ 44.4		$ 42.2
Fair Value Option Changes In Fair Value Gain Loss	0.3	4.2	2.2	2.5
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	88.4		88.4		87.3
Available For Sale Securities Gross Unrealized Gains			1.5		1.3
Available For Sale Securities Gross Unrealized Losses 1			(0.1)		(0.3)
Available For Sale Securities Amortized Cost	87.0		87.0		86.3
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	6.7		6.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	38.9		38.9
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	12.5		12.5
Available For Sale Securities Debt Maturities Within One Year Fair Value	30.3		30.3
Available For Sale Securities Debt Maturities Fair Value	88.4		88.4
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	12.3		12.3
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	30.2		30.2
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	38.2		38.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	6.3		6.3
Available For Sale Securities Debt Maturities Amortized Cost	87.0		87.0
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	132.8		132.8		129.5
Western Massachusetts Electric Company [Member] \| US Government Debt Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure					6.0
Available For Sale Securities Gross Unrealized Losses 1					0
Available For Sale Securities Amortized Cost					6.0
Western Massachusetts Electric Company [Member] \| 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					6.0
US Government Debt Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	13.4		13.4		17.8
Available For Sale Securities Gross Unrealized Gains			0.3		0.2
Available For Sale Securities Gross Unrealized Losses 1			0		(0.1)
Available For Sale Securities Amortized Cost	13.1		13.1		17.7
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	13.4		13.4		17.8
Western Massachusetts Electric Company [Member] \| Corporate Bond Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	5.9		5.9		15.6
Available For Sale Securities Gross Unrealized Losses 1			0		0
Available For Sale Securities Amortized Cost	5.9		5.9		15.6
Western Massachusetts Electric Company [Member] \| Corporate Bond Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	5.9		5.9		15.6
Corporate Bond Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	13.1		13.1		22.5
Available For Sale Securities Gross Unrealized Gains			0.5		0.5
Available For Sale Securities Gross Unrealized Losses 1			0		(0.1)
Available For Sale Securities Amortized Cost	12.6		12.6		22.1
Corporate Bond Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	13.1		13.1		22.5
Western Massachusetts Electric Company [Member] \| Asset Backed Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	3.0		3.0		4.7
Available For Sale Securities Gross Unrealized Losses 1			(0.1)		(0.1)
Available For Sale Securities Amortized Cost	3.1		3.1		4.8
Western Massachusetts Electric Company [Member] \| Asset Backed Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	3.0		3.0		4.7
Asset Backed Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	9.7		9.7		11.6
Available For Sale Securities Gross Unrealized Gains			0.4		0.4
Available For Sale Securities Gross Unrealized Losses 1			(0.1)		(0.1)
Available For Sale Securities Amortized Cost	9.4		9.4		11.3
Asset Backed Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	9.7		9.7		11.6
Western Massachusetts Electric Company [Member] \| Municipal Bonds Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	25.8		25.8		15.4
Available For Sale Securities Gross Unrealized Losses 1			0		0
Available For Sale Securities Amortized Cost	25.8		25.8		15.4
Western Massachusetts Electric Company [Member] \| Municipal Bonds Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	25.8		25.8		15.4
Municipal Bonds Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	26.5		26.5		16.1
Available For Sale Securities Gross Unrealized Gains			0.1		0
Available For Sale Securities Gross Unrealized Losses 1			0		0
Available For Sale Securities Amortized Cost	26.4		26.4		16.1
Municipal Bonds Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	26.5		26.5		16.1
Western Massachusetts Electric Company [Member] \| Other Debt Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	22.4		22.4		15.4
Available For Sale Securities Gross Unrealized Losses 1			0		0
Available For Sale Securities Amortized Cost	22.4		22.4		15.4
Western Massachusetts Electric Company [Member] \| Other Debt Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	21.3		21.3		15.1
Other Debt Securities Member
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	25.7		25.7		19.3
Available For Sale Securities Gross Unrealized Gains			0.2		0.2
Available For Sale Securities Gross Unrealized Losses 1			0		0
Available For Sale Securities Amortized Cost	25.5		25.5		19.1
Other Debt Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	23.6		23.6		17.5
Western Massachusetts Electric Company [Member] \| 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	1.1		1.1		0.3
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	2.1		2.1		1.8
Mutual Funds [Member] \| Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	44.4		44.4		42.2
Western Massachusetts Electric Company [Member] \| Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	1.1		1.1		0.3
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	46.5		46.5		44.0
Western Massachusetts Electric Company [Member] \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	56.0		56.0		56.8
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	86.3		86.3		85.5
Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	57.1		57.1		57.1
Available For Sale Securities Gross Unrealized Losses 1			(0.1)		(0.1)
Available For Sale Securities Amortized Cost	57.2		57.2		57.2
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	1.9		1.9
Available For Sale Securities Debt Maturities After Ten Years Fair Value	20.4		20.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	6.8		6.8
Available For Sale Securities Debt Maturities Within One Year Fair Value	28.0		28.0
Available For Sale Securities Debt Maturities Fair Value	57.1		57.1
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	6.7		6.7
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	28.0		28.0
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	20.5		20.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	2.0		2.0
Available For Sale Securities Debt Maturities Amortized Cost	57.2		57.2
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	$ 57.1		$ 57.1		$ 57.1