Summary of significant accounting policies - Nuclear decommissioning funds (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Nuclear decommissioning funds
Additional contribution to external trust funds	$ 2,643,000	$ 0
Additional amount collected for nuclear decommissioning	$ 8,350,000	$ 9,750,000
Percentage of decommissioning fund classified as equity	69.00%	71.00%
Percentage of decommissioning funds classified as fixed income securities	31.00%	29.00%
External and Internal Trust Funds:
Purchases	$ 204,500,000	$ 675,153,000	$ 578,610,000
Fair Value	$ 540,716,000	659,910,000
Average annualized rate of return over the past ten years	5.90%
Average annualized rate of return since inception	5.80%
External Trust Funds
External and Internal Trust Funds:
Cost		427,476,000	409,214,000
Purchases	$ 204,500,000	675,153,000
Net Proceeds	(209,963,000)	(656,891,000)
Unrealized Gain(Loss)	118,703,000	232,434,000
Fair Value	540,716,000	659,910,000
Net realized gains or losses, interest income and dividends, contributions and fees	5,463,000	18,261,000
Internal Funds
External and Internal Trust Funds:
Cost		115,770,000	101,114,000
Purchases	76,207,000	204,150,000
Net Proceeds	(69,823,000)	(189,494,000)
Unrealized Gain(Loss)	16,254,000	44,916,000
Fair Value	138,408,000	160,686,000
Net realized gains or losses, interest income and dividends, contributions and fees	6,384,000	14,656,000
Equity | External Trust Funds
External and Internal Trust Funds:
Cost		223,336,000	212,387,000
Purchases	9,255,000	50,309,000
Net Proceeds	(3,655,000)	(39,360,000)
Unrealized Gain(Loss)	131,572,000	230,710,000
Fair Value	360,508,000	454,046,000
Equity | Internal Funds
External and Internal Trust Funds:
Cost		68,914,000	50,647,000
Purchases	0	0
Net Proceeds	10,005,000	18,267,000
Unrealized Gain(Loss)	18,995,000	44,735,000
Fair Value	97,914,000	113,649,000
Debt | External Trust Funds
External and Internal Trust Funds:
Cost		204,935,000	196,810,000
Purchases	191,958,000	583,003,000
Net Proceeds	(203,907,000)	(574,878,000)
Unrealized Gain(Loss)	(12,869,000)	1,724,000
Fair Value	180,117,000	206,659,000
Debt | Internal Funds
External and Internal Trust Funds:
Cost		46,856,000	50,467,000
Purchases	76,207,000	204,150,000
Net Proceeds	(79,828,000)	(207,761,000)
Unrealized Gain(Loss)	(2,741,000)	181,000
Fair Value	40,494,000	47,037,000
Other | External Trust Funds
External and Internal Trust Funds:
Cost		(795,000)	$ 17,000
Purchases	3,287,000	41,841,000
Net Proceeds	(2,401,000)	(42,653,000)
Unrealized Gain(Loss)	0	0
Fair Value	$ 91,000	$ (795,000)