ESPP Discounted Purchase Price First Six Months

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- Definition

ESPP Discounted Purchase Price Second Six Months

+ References

No definition available.

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- Definition

Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount

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No definition available.

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Effective Tax Rate Reconciliation, Percent, Tax Effect on Share-Based Payments

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- Definition

Shares remaining available for future issuance under the ESPP, a noncompensatory plan.

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No definition available.

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- Definition

Percentage of Closing Market Price for ESPP Purchased By Employees at the End of Successive Six-Month Offering Period

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No definition available.

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- Definition

Period of Graded Vesting Schedule Under Long-Term Incentive Program

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- Definition

As of the balance sheet date, the number of shares into which expected to vest awards outstanding can be converted under the option plan.

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No definition available.

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- References

No definition available.

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- Definition

Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (j)
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- Definition

Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB
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- Definition

Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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- Definition

Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- 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.

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No definition available.

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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SHARE-BASED PAYMENTS - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in incentive plan (up to) (in shares)		8,000,000
Shares available for issuance under incentive plan (in shares)		2,692,350	3,005,010
Shares outstanding (in shares)		322,158
Weighted average grant date fair value of restricted stock units (in dollars per share)		$ 48.96
Total unrecognized compensation expense		$ 13.9
Period for recognition (in years)		1 year 9 months 4 days
Tax effect on total share-based payments (in percentage)		40.00%
Excess tax benefit		$ 19.1
Proceeds from excess tax benefit			$ 9.5	$ 9.5
Weighted average remaining contractual life		2 years
Cash received from stock options exercised		$ 1.3
Tax benefit realized from stock options		$ 0.5
Percentage of ESPP purchased by employees at the end of successive six-month offering period (in percentage)		95.00%
Number of shares purchased at discounted prices (in shares)		16,014	33,715
ESPP discounted purchase price for first six months (in dollars per share)		$ 51.11	$ 52.80
ESPP discounted purchase price for second six months (in dollars per share)			$ 47.23
Employee stock purchase plan available for future issuances (in shares)			743,260
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)	$ 42.27	54.67	$ 54.57
Shares outstanding (in shares)			469,772
Weighted average shares outstanding (in dollars per share)		$ 53.47	$ 48.58
Restricted stock units available (in shares)		402,263
Restricted stock unit fully vested (in shares)		402,112
Additional restricted stock unit expected to vest (in shares)		306,050
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)		$ 53.64	$ 55.04	$ 43.40
Shares outstanding (in shares)		301,363	528,428
Weighted average shares outstanding (in dollars per share)		$ 51.52	$ 46.30
Restricted stock units available (in shares)		423,025
Weighted average grant date fair value of restricted stock units (in dollars per share)		$ 45.94
Restricted stock unit fully vested (in shares)		221,571
The Connecticut Light And Power Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense		$ 5.1
Period for recognition (in years)		1 year 9 months 4 days
NSTAR Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense		$ 3.8
Period for recognition (in years)		1 year 9 months 4 days
Expiration period		10 years
Public Service Company Of New Hampshire [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense		$ 2.0
Period for recognition (in years)		1 year 9 months 4 days
Western Massachusetts Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense		$ 0.9
Period for recognition (in years)		1 year 9 months
Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of graded vesting schedule under long-term incentive program		3 years
Board Members [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of graded vesting schedule under long-term incentive program		1 year