Summary of significant accounting policies - Nuclear decommissioning funds (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Nuclear decommissioning funds
Additional contribution to external trust funds	$ 0	$ 0
Additional amount collected for nuclear decommissioning	$ 9,750,000	$ 4,750,000
Percentage of decommissioning fund classified as equity	71.00%	64.00%
Percentage of decommissioning funds classified as fixed income securities	29.00%	36.00%
External and Internal Trust Funds:
Purchases	$ 675,153,000	$ 578,610,000	$ 374,338,000
Fair Value	$ 659,910,000	598,181,000
Average annualized rate of return over the past ten years	9.40%
Average annualized rate of return since inception	6.80%
External Trust Funds
External and Internal Trust Funds:
Cost		409,214,000	386,528,000
Purchases	$ 675,153,000	578,610,000
Net Proceeds	(656,891,000)	(555,923,000)
Unrealized Gain(Loss)	232,434,000	188,966,000
Fair Value	659,910,000	598,181,000
Net realized gains or losses, interest income and dividends, contributions and fees	18,261,000	22,686,000
Internal Funds
External and Internal Trust Funds:
Cost		101,114,000	91,409,000
Purchases	204,150,000	132,710,000
Net Proceeds	(189,494,000)	(123,004,000)
Unrealized Gain(Loss)	44,916,000	36,184,000
Fair Value	160,686,000	137,299,000
Net realized gains or losses, interest income and dividends, contributions and fees	14,656,000	9,705,000
Equity | External Trust Funds
External and Internal Trust Funds:
Cost		212,387,000	212,585,000
Purchases	50,309,000	27,707,000
Net Proceeds	(39,360,000)	(16,444,000)
Unrealized Gain(Loss)	230,710,000	177,866,000
Fair Value	454,046,000	401,714,000
Equity | Internal Funds
External and Internal Trust Funds:
Cost		50,647,000	47,062,000
Purchases	0	0
Net Proceeds	18,267,000	3,586,000
Unrealized Gain(Loss)	44,735,000	33,665,000
Fair Value	113,649,000	84,313,000
Debt | External Trust Funds
External and Internal Trust Funds:
Cost		196,810,000	174,645,000
Purchases	583,003,000	536,147,000
Net Proceeds	(574,878,000)	(523,926,000)
Unrealized Gain(Loss)	1,724,000	11,100,000
Fair Value	206,659,000	197,966,000
Debt | Internal Funds
External and Internal Trust Funds:
Cost		50,467,000	44,347,000
Purchases	204,150,000	132,710,000
Net Proceeds	(207,761,000)	(126,590,000)
Unrealized Gain(Loss)	181,000	2,519,000
Fair Value	47,037,000	52,986,000
Other | External Trust Funds
External and Internal Trust Funds:
Cost		17,000	$ (702,000)
Purchases	41,841,000	14,756,000
Net Proceeds	(42,653,000)	(15,553,000)
Unrealized Gain(Loss)	0	0
Fair Value	$ (795,000)	$ (1,499,000)