Related Parties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011 The Connecticut Light And Power Company [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco		$ 25.0	$ 3.8	$ 5.5
Contributions To Nu Foundation	$ 2.0