Income taxes: (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income taxes:
Current period income tax expense	$ 0
Current income tax liability	0
Difference between statutory federal income tax rate on income before income taxes and effective income tax rate
Statutory federal income tax rate (as a percent)	35.00%	35.00%	35.00%
Patronage exclusion (as a percent)	(33.30%)	(32.70%)	(32.60%)
Tax credits (as a percent)	0.00%	0.00%	0.00%
Other (as a percent)	(1.70%)	(2.30%)	(2.40%)
Effective income tax rate (as a percent)	0.00%	0.00%	0.00%
Deferred tax assets
Net operating losses	29,724,000	29,724,000
Tax credits (alternative minimum tax and other)	1,478,000	1,535,000
Accounting for Rocky Mountain transactions	348,136,000	347,867,000
Other assets	23,233,000	18,859,000
Deferred tax assets, gross	402,571,000	397,985,000
Less: Valuation allowance	(31,202,000)	(31,259,000)
Net deferred tax assets	371,369,000	366,726,000
Deferred tax liabilities
Depreciation	448,350,000	465,598,000
Accounting for Rocky Mountain transactions	153,173,000	157,145,000
Other liabilities	60,354,000	44,508,000
Deferred tax liabilities, gross	661,877,000	667,251,000
Net deferred tax liabilities	290,508,000	300,525,000
Less: Patronage exclusion	$ (290,508,000)	$ (300,525,000)