Income taxes - Statutory federal and effective income tax rate and components of deferred tax assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
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%	21.00%
Patronage exclusion	(21.00%)	(21.00%)	(21.00%)
Effective income tax rate	0.00%	0.00%	0.00%
Deferred tax assets
Obligation related to asset retirements	$ 331,311,000	$ 286,546,000
Advance payments	175,077,000	112,394,000
Other regulatory liabilities	54,611,000	27,651,000
Other assets	24,838,000	17,424,000
Deferred tax assets	585,837,000	444,015,000
Less: Valuation allowance	0	0
Net deferred tax assets	585,837,000	444,015,000
Deferred tax liabilities
Fixed assets and intangibles	(40,655,000)	(132,520,000)
Right-of-Use assets-finance leases	(77,923,000)	(77,923,000)
Other regulatory asset	(338,291,000)	(207,186,000)
Other liabilities	(17,984,000)	(19,233,000)
Deferred tax liabilities	(474,853,000)	(436,862,000)
Net deferred tax assets/(liabilities)	110,984,000	7,153,000
Less: Patronage exclusion	(110,984,000)	(7,153,000)
Net deferred taxes	$ 0	$ 0