Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, the maximum amount borrowed at any time during the period.

+ References

No definition available.

+ Details

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Period Type:	duration

- Details

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SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Issuances (Details) - Senior Notes - USD ($)	1 Months Ended
	Oct. 31, 2017	Aug. 31, 2017	Sep. 30, 2017	May 31, 2017	Mar. 31, 2017
Series N First Mortgage Bonds Due 2027
Debt Instrument [Line Items]
Debt instrument, face amount			$ 75,000,000
Interest rate stated percentage			3.02%
Eversource Parent \| Series K Senior Notes Due 2022
Debt Instrument [Line Items]
Debt instrument, face amount					$ 300,000,000
Interest rate stated percentage					2.75%
The Connecticut Light And Power Company \| 2017 Series A First and Refunding Mortgage Bonds Due 2027
Debt Instrument [Line Items]
Debt instrument, face amount					$ 300,000,000
Interest rate stated percentage					3.20%
The Connecticut Light And Power Company \| 2014 Series A First and Refunding Mortgage Bonds Due in 2044
Debt Instrument [Line Items]
Debt instrument, face amount		$ 225,000,000
Interest rate stated percentage		4.30%
Outstanding principal amount		$ 475,000,000
NSTAR Electric Company \| Unsecured Notes Due 2027
Debt Instrument [Line Items]
Debt instrument, face amount				$ 350,000,000
Interest rate stated percentage				3.20%
Subsequent Event \| Eversource Parent \| Series K Senior Notes Due 2022
Debt Instrument [Line Items]
Debt instrument, face amount	$ 450,000,000
Interest rate stated percentage	2.75%
Outstanding principal amount	$ 750,000,000
Subsequent Event \| Eversource Parent \| 2017 Series L Senior Notes, Due in 2024
Debt Instrument [Line Items]
Debt instrument, face amount	$ 450,000,000
Interest rate stated percentage	2.90%
Subsequent Event \| NSTAR Electric Company \| Unsecured Notes Due 2027
Debt Instrument [Line Items]
Debt instrument, face amount	$ 350,000,000
Interest rate stated percentage	3.20%
Outstanding principal amount	$ 700,000,000