SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 196.8	$ 200.6
Uncollectible Hardship	126.3	119.9
The Connecticut Light And Power Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	77.6	86.4
Uncollectible Hardship	64.6	67.7
NSTAR Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	55.7	54.8
Uncollectible Hardship	32.3	26.2
Public Service Company Of New Hampshire
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10.6	9.9
Uncollectible Hardship	0.0	0.0
Western Massachusetts Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	17.0	15.5
Uncollectible Hardship	$ 11.3	$ 9.9