Income taxes: - Statutory federal and effective income tax rate and components of deferred tax assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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)	(34.70%)	(34.70%)	(34.40%)
Other (as a percent)	(0.30%)	(0.30%)	(0.60%)
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)	599,000	1,198,000
Accounting for Rocky Mountain transactions	349,127,000	348,779,000
Other assets	109,793,000	107,527,000
Deferred tax assets, gross	489,243,000	487,228,000
Less: Valuation allowance	(29,724,000)	(29,724,000)
Net deferred tax assets	459,519,000	457,504,000
Deferred tax liabilities
Depreciation	435,570,000	426,173,000
Accounting for Rocky Mountain transactions	170,402,000	167,907,000
Other liabilities	123,121,000	131,454,000
Deferred tax liabilities, gross	729,093,000	725,534,000
Net deferred tax liabilities	269,574,000	268,030,000
Less: Patronage exclusion	$ (269,574,000)	$ (268,030,000)