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-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

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=16383099&loc=d3e24584-111560

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale which are intended to be held for greater than one year or the normal operating cycle, whichever is longer and that are pledged to one or more secured parties who have the right to buy, sell, or re-pledge the collateral.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(b))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph b

-Article 4

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher OTS

-Name Federal Regulation (FR)

-Number Title 12

-Section 563c.102

-Paragraph 6

-Subparagraph a

-Chapter V

-Subsection I

-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

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MARKETABLE SECURITIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$ 51.3		$ 51.3		$ 47.0
Fair Value, Option, Changes in Fair Value, Gain (Loss)	0.1	(0.5)	4.3	2.7
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	440.0		440.0		340.4
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	59.0		59.0
Available For Sale Securities Debt Maturities After Ten Years Fair Value	103.3		103.3
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	74.5		74.5
Available For Sale Securities Debt Maturities Within One Year Fair Value	102.4		102.4
Available For Sale Securities Debt Maturities Fair Value	339.2		339.2
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	73.7		73.7
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	102.4		102.4
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	99.4		99.4
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	57.9		57.9
Available For Sale Securities Debt Maturities Amortized Cost	333.4		333.4
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	596.4		596.4		492.3
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	3.6		3.6
Available For Sale Securities Debt Maturities After Ten Years Fair Value	5.4		5.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	23.6		23.6
Available For Sale Securities Debt Maturities Within One Year Fair Value	25.1		25.1
Available For Sale Securities Debt Maturities Fair Value	57.7		57.7
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	23.7		23.7
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	25.1		25.1
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	5.4		5.4
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	3.6		3.6
Available For Sale Securities Debt Maturities Amortized Cost	57.8		57.8
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	57.7		57.7		57.7
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	334.6		334.6		252.7
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	4.8		4.8		5.2
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	261.8		261.8		239.6
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	52.9		52.9		52.5
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	81.6		81.6		69.9
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	16.6		16.6		18.7
Corporate Bond Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	46.5		46.5		33.0
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	12.6		12.6		7.0
Asset Backed Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	29.9		29.9		28.5
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	10.5		10.5		10.9
Municipal Bonds Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	89.1		89.1		93.8
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	9.7		9.7		11.6
Other Debt Securities Member \| Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	14.7		14.7		14.4
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	3.5		3.5		4.3
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	77.4		77.4		40.2
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	4.8		4.8		5.2
Mutual Funds [Member] \| Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	257.2		257.2		212.5
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	339.2		339.2		279.8
Available-for-sale Securities, Gross Unrealized Gains	6.5		6.5		13.3
Available-for-sale Securities, Gross Unrealized Losses	(0.7)		(0.7)		(0.1)
Available For Sale Securities Amortized Cost	333.4		333.4		266.6
Debt Securities \| Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	57.7		57.7		57.7
Available-for-sale Securities, Gross Unrealized Gains	0		0		0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.1)		(0.1)		(0.1)
Available For Sale Securities Amortized Cost	57.8		57.8		57.7
Equity Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	205.9		205.9		165.5
Available-for-sale Securities, Gross Unrealized Gains	42.2		42.2		20.0
Available For Sale Securities Amortized Cost	$ 163.7		$ 163.7		$ 145.5