Plant acquisition: (Details) (USD $)	12 Months Ended						12 Months Ended		0 Months Ended
	Dec. 31, 2012 MW	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Smith item MW	Dec. 31, 2011 Smith	Apr. 08, 2011 Smith	Dec. 31, 2012 Smith Deferred costs associated with cost to acquire Murray	Dec. 31, 2011 Smith Deferred costs associated with cost to acquire Murray	Apr. 08, 2011 Smith Term loan	Apr. 08, 2011 Smith Commercial paper
Plant acquisitions
Percentage of entity acquired						100.00%
Number of natural gas-fired combined cycle units that the Smith Energy Facility contains				2
Aggregate summer planning reserve generation capacity of the natural gas-fired combined cycle units acquired (in megawatts)	6,844			1,250
Acquisition related costs					$ 1,962,000
Cash outlay for acquisition of business						532,255,000
Term loan									260,000,000	272,255,000
Term of debt									3 years
Recognized fair value amounts of identifiable assets acquired and liabilities assumed:
Property, plant and equipment						456,700,000
Inventory						34,000,000
Other current assets						4,600,000
Power purchase and sale agreement						40,400,000
Emission credits						200,000
Current liabilities						(5,600,000)
Total identifiable net assets						530,300,000
Non-member sales	120,102,000	166,040,000	1,478,000	119,723,000	165,389,000
Amortization period							30 years
Deferred costs, net							$ 17,582,000	$ 3,536,000