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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Total Provision for Uncollectible Accounts $ 211.3   $ 211.3   $ 195.7
Uncollectible Hardship 126.8   126.8   122.5
Bad debt expense associated with accounts receivable 9.5 $ (0.3) $ 29.3 $ 15.7  
The Connecticut Light and Power Company          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Period of accounts receivable recoverable under financial or medical duress     180 days    
Total Provision for Uncollectible Accounts 80.4   $ 80.4   78.9
Uncollectible Hardship 64.5   64.5   65.5
Bad debt expense associated with accounts receivable 3.8 (4.7) $ 7.7 (1.9)  
Yankee Gas Services Company          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Period of accounts receivable recoverable under financial or medical duress     90 days    
NSTAR Electric Company          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Total Provision for Uncollectible Accounts 77.2   $ 77.2   69.7
Uncollectible Hardship 44.0   44.0   40.3
Bad debt expense associated with accounts receivable 3.8 0.4 11.3 6.9  
Public Service Company of New Hampshire          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Total Provision for Uncollectible Accounts 11.5   11.5   10.5
Uncollectible Hardship 0.0   0.0   $ 0.0
Bad debt expense associated with accounts receivable $ 1.5 $ 1.9 $ 3.2 $ 3.6