SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 203.2	$ 200.6
Uncollectible Hardship	118.0	119.9
The Connecticut Light And Power Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	88.7	86.4
Uncollectible Hardship	69.8	67.7
NSTAR Electric Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	52.6	54.8
Uncollectible Hardship	23.7	26.2
Public Service Company Of New Hampshire [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10.4	9.9
Uncollectible Hardship	0.0	0.0
Western Massachusetts Electric Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	14.2	15.5
Uncollectible Hardship	$ 8.2	$ 9.9