Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 175.3	$ 171.3
Uncollectible Hardship Accounts Receivable Reserve	91.5	81.2
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	84.3	82.0
Uncollectible Hardship Accounts Receivable Reserve	74.0	67.3
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	40.7	41.7
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	7.7	7.4
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	9.9	10.0
Uncollectible Hardship Accounts Receivable Reserve	$ 6.2	$ 5.5