Provision for Uncollectible Accounts (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 38.2	$ 39.8
The Connecticut Light and Power Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	16.4	17.2
Public Service Company of New Hampshire [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	7.5	6.8
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 5.2	$ 6.0