Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 190.7	$ 175.3
Uncollectible Hardship Accounts Receivable Reserve	118.5	91.5
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	79.5	84.3
Uncollectible Hardship Accounts Receivable Reserve	68.1	74.0
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	52.6	40.7
Uncollectible Hardship Accounts Receivable Reserve	25.3
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	8.7	7.7
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	14.0	9.9
Uncollectible Hardship Accounts Receivable Reserve	$ 7.4	$ 6.2