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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6956420&loc=d3e24584-111560

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 44.3		$ 41.1
Fair Value Option Changes In Fair Value Gain Loss	(3.2)	(1.9)
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	92.5		90.2
Available-for-sale Securities, Gross Unrealized Gains	2.1		2.0
Available-for-sale Securities, Gross Unrealized Losses	(0.1)		(0.2)
Available For Sale Securities Amortized Cost	90.5		88.4
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	12.5
Available For Sale Securities Debt Maturities After Ten Years Fair Value	27.8
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	33.8
Available For Sale Securities Debt Maturities Within One Year Fair Value	18.4
Available For Sale Securities Debt Maturities Fair Value	92.5
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	33.5
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	18.4
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	26.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	12.0
Available For Sale Securities Debt Maturities Amortized Cost	90.5
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	136.8		131.3
Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	57.4		57.1
Available-for-sale Securities, Gross Unrealized Gains	0.1		0
Available-for-sale Securities, Gross Unrealized Losses	(0.1)		(0.2)
Available For Sale Securities Amortized Cost	57.4		57.3
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	6.6
Available For Sale Securities Debt Maturities After Ten Years Fair Value	9.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	27.4
Available For Sale Securities Debt Maturities Within One Year Fair Value	14.0
Available For Sale Securities Debt Maturities Fair Value	57.4
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	27.4
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	13.9
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	9.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	6.6
Available For Sale Securities Debt Maturities Amortized Cost	57.4
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	57.4		57.1
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	50.5		42.9
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	3.1		0.1
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	86.3		88.4
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	54.3		57.0
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	22.4		11.1
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	19.3		8.0
Corporate Bond Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	15.4		16.5
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	7.3		9.1
Asset Backed Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	22.9		25.9
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	5.1		7.9
Municipal Bonds Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	13.6		16.1
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	12.8		15.4
Other Debt Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	12.0		18.8
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	9.8		16.6
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	6.2		1.8
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	3.1		0.1
Mutual Funds [Member] \| Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	44.3		41.1
Mutual Funds [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	$ 0		$ 0