Summary of significant accounting policies: (Details 5) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Electric plant, construction and related agreements
Depreciation expense	$ 171,240,000	$ 164,901,000	$ 165,603,000
Electric plant
Construction rates for allowance of funds (as a percent)	4.93%	5.12%	5.55%
Restricted cash
Restricted cash balance	35,131,000	9,109,000
Long-term restricted cash	34,975,000	8,953,000
Restricted short-term investments
Deposit with Rural Utilities Service in Cushion of Credit Account	272,686,000	64,671,000
Guaranteed interest rate on deposit with Rural Utilities Service (as a percent)	5.00%	5.00%
Inventories
Fossil fuels inventories	124,359,000	94,491,000
Spare parts	161,808,000	169,458,000
Deferred charges and other assets
Remaining amortization period for debt issuance costs, minimum	1 year
Remaining amortization period for debt issuance costs, maximum	37 years
Deferred credits and liabilities
Member power bill prepayments	114,718,000
Member power bill prepayments classified as current liabilities	82,405,000	65,079,000
Member power bill prepayments classified as deferred credits and other liabilities	$ 32,313,000	$ 40,853,000
Steam production
Electric plant, construction and related agreements
Annual depreciation rates (as a percent)	1.82%	1.85%	1.88%
Steam production | Minimum
Electric plant, construction and related agreements
Range of Useful Life	49 years
Steam production | Maximum
Electric plant, construction and related agreements
Range of Useful Life	65 years
Nuclear production
Electric plant, construction and related agreements
Annual depreciation rates (as a percent)	1.54%	1.54%	1.45%
Nuclear production | Minimum
Electric plant, construction and related agreements
Range of Useful Life	37 years
Nuclear production | Maximum
Electric plant, construction and related agreements
Range of Useful Life	60 years
Hydro production
Electric plant, construction and related agreements
Range of Useful Life	50 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
Other production
Electric plant, construction and related agreements
Annual depreciation rates (as a percent)	2.55%	2.74%	2.74%
Other production | Minimum
Electric plant, construction and related agreements
Range of Useful Life	27 years
Other production | Maximum
Electric plant, construction and related agreements
Range of Useful Life	33 years
Transmission
Electric plant, construction and related agreements
Range of Useful Life	36 years
Annual depreciation rates (as a percent)	2.75%	2.75%	2.75%
General | Minimum
Electric plant, construction and related agreements
Range of Useful Life	3 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
General | Maximum
Electric plant, construction and related agreements
Range of Useful Life	50 years
Annual depreciation rates (as a percent)	33.33%	33.33%	33.33%