Income taxes: Statutory federal and effective income tax rate and components of deferred tax assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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%)	(34.40%)	(33.30%)
Other (as a percent)	(0.30%)	(0.60%)	(1.70%)
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)	1,198,000	1,275,000
Accounting for Rocky Mountain transactions	348,779,000	348,460,000
Other assets	107,527,000	100,203,000
Deferred tax assets, gross	487,228,000	479,662,000
Less: Valuation allowance	(29,724,000)	(30,999,000)
Net deferred tax assets	457,504,000	448,663,000
Deferred tax liabilities
Depreciation	426,173,000	444,139,000
Accounting for Rocky Mountain transactions	167,907,000	165,462,000
Other liabilities	131,454,000	112,732,000
Deferred tax liabilities, gross	725,534,000	722,333,000
Net deferred tax liabilities	268,030,000	273,670,000
Less: Patronage exclusion	$ (268,030,000)	$ (273,670,000)