Provision for Uncollectible Accounts (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 93.1	$ 34.9
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	11.8	14.8
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	7.9	7.2
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	3.7	4.6
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable Line Items
Allowance For Doubtful Accounts Receivable Current	$ 44.7	$ 27.1