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Fuel, Materials and Supplies and Allowance Inventory (Details) (Public Service Company Of New Hampshire [Member], Emission Allowances [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Public Service Company Of New Hampshire [Member] \| Emission Allowances [Member]
Other Inventory, Capitalized Costs	$ 0.8	$ 7.1
Other Inventory, Noncurrent	19.4	18.2
Amortization of Power Contracts Emission Credits	$ 5.1	$ 6.6	$ 7.6

The expense charged against earnings for the periodic recognition of capitalized costs related to purchasing power, fuel, or emission credits. These items may be purchased in one accounting period but are deducted from earnings over time through amortization.

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