Income taxes - Statutory federal and effective income tax rate and components of deferred tax assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	21.00%	21.00%	35.00%
Patronage exclusion	(21.00%)	(20.80%)	(34.10%)
AMT credit monetization	0.00%	0.00%	2.20%
Other	0.00%	(0.20%)	(0.90%)
Effective income tax rate	0.00%	0.00%	2.20%
Deferred tax assets
Net operating losses	$ 1,123,000	$ 3,830,000
Tax credits (alternative minimum tax and other)	0	0
Accounting for Rocky Mountain transactions	231,844,000	231,543,000
Advance payments	74,482,000	46,708,000
Other assets	88,821,000	82,655,000
Deferred tax assets	396,270,000	364,736,000
Less: Valuation allowance	(1,123,000)	(3,830,000)
Net deferred tax assets	395,147,000	360,906,000
Deferred tax liabilities
Depreciation	258,724,000	268,039,000
Accounting for Rocky Mountain transactions	118,021,000	116,226,000
Other liabilities	68,035,000	75,691,000
Deferred tax liabilities	444,780,000	459,956,000
Net deferred tax liabilities	49,633,000	99,050,000
Less: Patronage exclusion	(49,633,000)	(99,050,000)
Net deferred taxes	$ 0	0
Alternative Minimum Tax Credits
Difference between statutory federal income tax rate on income before income taxes and effective income tax rate
Tax benefit reflected in effective income tax rate		$ 1,117,000