Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 171.3	$ 165.5
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	82.0	77.6
Uncollectible Hardship Accounts Receivable Reserve	67.3	65.2
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	41.7	44.1
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	7.4	6.8
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	10.0	8.5
Uncollectible Hardship Accounts Receivable Reserve	5.5	4.7
Yankee Gas Services Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Uncollectible Hardship Accounts Receivable Reserve	$ 8.4	$ 6.4