Income taxes: (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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)	(32.70%)	(32.60%)	(32.90%)
Tax credits (as a percent)	0.00%	0.00%	0.00%
Other (as a percent)	(2.30%)	(2.40%)	(2.10%)
Effective income tax rate (as a percent)	0.00%	0.00%	0.00%
Deferred tax assets
Net operating losses	$ 29,724	$ 29,724
Tax credits (alternative minimum tax and other)	1,535	1,535
Accounting for Rocky Mountain transactions	347,867	334,423
Other assets	18,859	14,312
Deferred tax assets, gross	397,985	379,994
Less: Valuation allowance	(31,259)	(31,259)
Net deferred tax assets	366,726	348,735
Deferred tax liabilities
Depreciation	465,598	485,721
Accounting for Rocky Mountain transactions	157,145	161,214
Other liabilities	44,508	39,109
Deferred tax liabilities, gross	667,251	686,044
Net deferred tax liabilities	300,525	337,309
Less: Patronage exclusion	$ (300,525)	$ (337,309)