Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 34.9	$ 39.8
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	14.8	17.2
Uncollectible Hardship Accounts Receivable Reserve	68.6	65.0
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	7.2	6.8
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	4.6	6.0
Uncollectible Hardship Accounts Receivable Reserve	5.4	6.9
Yankee Gas Services Company [Member]
Accounts Notes And Loans Receivable Line Items
Uncollectible Hardship Accounts Receivable Reserve	$ 6.8	$ 7.5