Operating Expenses (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fuel [Line Items]
Fuel Power Costs	$ 346.8	$ 307.9	$ 391.6
Public Service Company Of New Hampshire [Member]
Fuel [Line Items]
Fuel Power Costs	103.4	115.9	184.3
Yankee Gas Services Company [Member]
Fuel [Line Items]
Fuel Power Costs	145.9	191.3	206.4
NSTAR Gas Company [Member]
Fuel [Line Items]
Fuel Power Costs	$ 97.2