SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 207.7	$ 195.7
Uncollectible Hardship	123.7	122.5
Bad debt expense associated with accounts receivable	19.6		$ 15.9
The Connecticut Light And Power Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	82.8	78.9
Uncollectible Hardship	67.7	65.5
Bad debt expense associated with accounts receivable	3.9		2.8
NSTAR Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	72.6	69.7
Uncollectible Hardship	38.5	40.3
Bad debt expense associated with accounts receivable	7.5		6.5
Public Service Company Of New Hampshire
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10.9	10.5
Uncollectible Hardship	0.0	$ 0.0
Bad debt expense associated with accounts receivable	$ 1.7		$ 1.7