SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 208.1	$ 190.7
Uncollectible Hardship	126.5	118.5
The Connecticut Light And Power Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	85.3	79.5
Uncollectible Hardship	66.9	68.1
NSTAR Electric Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	59.9	52.6
Uncollectible Hardship	30.3	25.3
Public Service Company Of New Hampshire [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	9.9	8.7
Uncollectible Hardship	0.0	0.0
Western Massachusetts Electric Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	16.8	14.0
Uncollectible Hardship	$ 10.7	$ 7.4