Provision for Uncollectible Accounts (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 197.6	$ 190.7
Uncollectible Hardship Accounts Receivable Reserve	118.9	118.5
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	84.9	79.5
Uncollectible Hardship Accounts Receivable Reserve	71.3	68.1
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	51.5	52.6
Uncollectible Hardship Accounts Receivable Reserve	22.7	25.3
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	9.1	8.7
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	13.6	14.0
Uncollectible Hardship Accounts Receivable Reserve	$ 7.1	$ 7.4