Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 165.5	$ 115.7
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	77.6	83.5
Uncollectible Hardship Accounts Receivable Reserve	65.2	68.6
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	6.8	7.2
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	8.5	10.0
Uncollectible Hardship Accounts Receivable Reserve	4.7	5.4
Yankee Gas Services Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Uncollectible Hardship Accounts Receivable Reserve	6.4	6.8
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$ 44.1	$ 27.1