Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 187.4	$ 175.3
Uncollectible Hardship Accounts Receivable Reserve	92.3	91.5
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	86.6	84.3
Uncollectible Hardship Accounts Receivable Reserve	74.5	74.0
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	43.8	40.7
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	8.1	7.7
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	10.7	9.9
Uncollectible Hardship Accounts Receivable Reserve	$ 6.5	$ 6.2