Nuclear Fuel Disposal Cost Settlement (Details) (Nuclear fuel disposal cost settlement, Plant Hatch, USD $)	3 Months Ended
	Jun. 30, 2012	Apr. 05, 2012	Dec. 31, 2007
Nuclear fuel disposal cost settlement | Plant Hatch
Nuclear Fuel Disposal Cost Settlement
Settlement amount, entity share		$ 16,205,000	$ 17,980,000
Settlement amount, all parties		54,017,000	59,900,000
Reduction of operating expenses	9,679,000
Reduction to plant in service	$ 6,526,000