Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 185.8	$ 175.3
Uncollectible Hardship Accounts Receivable Reserve	86.6	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	64.9	74.0
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	46.4	40.7
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	9.4	7.7
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	13.2	9.9
Uncollectible Hardship Accounts Receivable Reserve	$ 7.7	$ 6.2