Plant acquisition: (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 08, 2011	Dec. 31, 2013 MW item	Dec. 31, 2012	Dec. 31, 2011
Plant acquisitions
Aggregate summer planning reserve generation capacity of the natural gas-fired combined cycle units acquired (in megawatts)		7,035
Recognized fair value amounts of identifiable assets acquired and liabilities assumed:
Non-member sales		$ 78,758,000	$ 120,102,000	$ 166,040,000
Smith
Plant acquisitions
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)		1,250
Acquisition related costs				1,962,000
Cash outlay for acquisition of business	532,255,000
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
Goodwill	0
Non-member sales		77,467,000	119,723,000	165,389,000
Smith | Deferred costs
Recognized fair value amounts of identifiable assets acquired and liabilities assumed:
Amortization period		30 years
Deferred costs, net		41,929,000	17,582,000	3,536,000
Smith | Term loan
Plant acquisitions
Term loan	260,000,000
Term of debt	3 years
Smith | Commercial paper
Plant acquisitions
Term loan	$ 272,255,000