SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 195.7	$ 200.6
Uncollectible Hardship	122.5	119.9
The Connecticut Light And Power Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	78.9	86.4
Uncollectible Hardship	65.5	67.7
NSTAR Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	69.7	70.3
Uncollectible Hardship	40.3	36.1
Public Service Company Of New Hampshire
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10.5	9.9
Uncollectible Hardship	$ 0.0	$ 0.0