Income taxes - Statutory federal and effective income tax rate and components of deferred tax assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
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
Net operating losses	$ 115,080,000	$ 0
Obligation related to asset retirements	345,879,000	331,311,000
Advance payments	183,833,000	175,077,000
Other regulatory liabilities	18,687,000	54,611,000
Other assets	30,373,000	24,838,000
Deferred tax assets	693,852,000	585,837,000
Less: Valuation allowance	0	0
Net deferred tax assets	693,852,000	585,837,000
Deferred tax liabilities
Fixed assets and intangibles	(140,095,000)	(40,655,000)
Right-of-use assets-finance leases	(77,923,000)	(77,923,000)
Other regulatory asset	(343,230,000)	(338,291,000)
Other liabilities	(15,352,000)	(17,984,000)
Deferred tax liabilities	(576,600,000)	(474,853,000)
Net deferred tax assets (liabilities)	117,252,000	110,984,000
Less: Patronage exclusion	(117,252,000)	(110,984,000)
Net deferred taxes	0	$ 0
Net operating loss carryforwards	$ 447,086,000