Provision for Uncollectible Accounts (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 34.0	$ 39.8
The Connecticut Light and Power Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	13.9	17.2
Public Service Company of New Hampshire [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	7.3	6.8
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 4.3	$ 6.0