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Share-Based Payments (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 8,000,000    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 3,005,010 3,112,020  
AllocatedShareBasedCompensationExpense $ 23,100,000 $ 24,600,000 $ 27,000,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 9,400,000 $ 10,300,000 10,700,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized $ 14,900,000    
Description Of Compensation Cost Not Yet Recognized Period For Recognition This cost is expected to be recognized ratably over a weighted-average period of 1.74 years for Eversource, and 1.73 years for each CL&P, NSTAR Electric, PSNH and WMECO.    
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number 469,772 1,024,729  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value $ 48.58 $ 38.14  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period 784,376    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value $ 37.21    
Share Based Compensation Arrangement By Share Based Payment Award Vested Outstanding Number 259,536    
Share Based Compensation Arrangement By Share Based Payment Award Expected To Vest 446,283    
Employee Service Share Based Compensation Stock Option Disclosures [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 171,872 351,616  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (179,744)    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 26.47 $ 26.69  
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice $ 26.90    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value $ 4,400,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value 4,200,000 $ 9,400,000  
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options 4,800,000    
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options $ 1,900,000    
Employee Service Share Based Compensation E S P P Disclosures [Abstract]      
Stock Issued During Period, Shares, Employee Stock Purchase Plans 33,715 40,779  
Employee Stock Purchase Plan Shares Remaining For Future Issuance 743,260 776,975  
ESPP Discounted Purchase Price First Six Months $ 52.8 $ 41.61  
ESPP Discounted Purchase Price Second Six Months 47.23 41.71  
Deferred Compensation Liability, Classified, Noncurrent [Abstract]      
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent 55,200,000 57,500,000 51,300,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits 3,900,000 4,500,000 4,400,000
The Connecticut Light And Power Company [Member]      
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
AllocatedShareBasedCompensationExpense 9,300,000 8,100,000 6,800,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 3,800,000 3,400,000 2,700,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 6,100,000    
Deferred Compensation Liability, Classified, Noncurrent [Abstract]      
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent 400,000 400,000 400,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits 1,500,000 2,100,000 2,500,000
NSTAR Electric Company [Member]      
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
AllocatedShareBasedCompensationExpense 5,800,000 7,400,000 7,500,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 2,400,000 3,100,000 3,000,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 3,800,000    
Deferred Compensation Liability, Classified, Noncurrent [Abstract]      
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits 1,000,000 300,000  
Public Service Company Of New Hampshire [Member]      
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
AllocatedShareBasedCompensationExpense 3,200,000 3,000,000 2,300,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 1,300,000 1,300,000 900,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 2,200,000    
Deferred Compensation Liability, Classified, Noncurrent [Abstract]      
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent 2,400,000 2,600,000 2,300,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits 700,000 900,000 1,000,000
Western Massachusetts Electric Company [Member]      
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
AllocatedShareBasedCompensationExpense 1,700,000 1,300,000 1,300,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 700,000 500,000 500,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 1,200,000    
Deferred Compensation Liability, Classified, Noncurrent [Abstract]      
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent 200,000 200,000 100,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits $ 300,000 $ 400,000 $ 500,000
Restricted Stock Units (RSUs) [Member]      
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 266,230    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (888,495)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (29,174)    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Ending Balance 729,308 1,380,747  
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Weighted Average Grant Date Fair Value Beginning Balance   $ 35.67  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 54.57 $ 42.27 $ 39.56
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Issued In Period Weighted Average Grant Date Fair Value 33.94    
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue 46.68    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Weighted Average Grant Date Fair Value Ending Balance $ 43.45    
Performance Shares [Member]      
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 172,543    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (4,604)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (15,155)    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Ending Balance 528,428 375,644  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 55.04 $ 43.40 $ 40.96
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Issued In Period Weighted Average Grant Date Fair Value 42.23    
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue 45.33    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Weighted Average Grant Date Fair Value Ending Balance $ 46.30 $ 42.20