Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 205.5	$ 190.7
Uncollectible Hardship Accounts Receivable Reserve	123.7	118.5
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	82.7	79.5
Uncollectible Hardship Accounts Receivable Reserve	66.2	68.1
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	56.9	52.6
Uncollectible Hardship Accounts Receivable Reserve	28.6	25.3
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	9.6	8.7
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	15.3	14.0
Uncollectible Hardship Accounts Receivable Reserve	$ 8.4	$ 7.4