Summary of significant accounting policies - Nuclear decommissioning funds (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nuclear decommissioning funds
Additional contribution to external trust funds	$ 0	$ 0
Additional amount collected for nuclear decommissioning	$ 4,750,000	$ 4,750,000
Percentage of decommissioning fund classified as equity	64.00%	60.00%
Percentage of decommissioning funds classified as fixed income securities	36.00%	40.00%
External and Internal Trust Funds:
Purchases	$ 374,338,000	$ 457,909,000	$ 450,113,000
Fair Value	$ 511,339,000	420,818,000
Average annualized rate of return over the past ten years	7.90%
Average annualized rate of return since inception	6.30%
External Trust Funds
External and Internal Trust Funds:
Cost		373,451,000	368,664,000
Purchases	$ 374,338,000	457,909,000
Net Proceeds	(361,261,000)	(453,122,000)
Unrealized Gain(Loss)	124,811,000	47,367,000
Fair Value	511,339,000	420,818,000
Net realized gains or losses, interest income and dividends, contributions and fees	13,078,000	4,786,000
Internal Funds
External and Internal Trust Funds:
Cost		82,677,000	77,238,000
Purchases	140,997,000	161,454,000
Net Proceeds	(132,266,000)	(156,015,000)
Unrealized Gain(Loss)	20,739,000	6,127,000
Fair Value	112,147,000	88,804,000
Net realized gains or losses, interest income and dividends, contributions and fees	8,732,000	689,000
Equity | External Trust Funds
External and Internal Trust Funds:
Cost		207,313,000	203,622,000
Purchases	11,950,000	12,186,000
Net Proceeds	(6,678,000)	(7,789,000)
Unrealized Gain(Loss)	119,263,000	49,475,000
Fair Value	331,848,000	257,494,000
Equity | Internal Funds
External and Internal Trust Funds:
Cost		44,295,000	43,698,000
Purchases	0	0
Net Proceeds	2,767,000	596,000
Unrealized Gain(Loss)	19,578,000	6,373,000
Fair Value	66,640,000	50,667,000
Debt | External Trust Funds
External and Internal Trust Funds:
Cost		166,023,000	164,901,000
Purchases	361,844,000	445,353,000
Net Proceeds	(353,222,000)	(443,712,000)
Unrealized Gain(Loss)	5,548,000	(2,108,000)
Fair Value	180,193,000	164,434,000
Debt | Internal Funds
External and Internal Trust Funds:
Cost		38,382,000	33,540,000
Purchases	140,997,000	161,454,000
Net Proceeds	(135,033,000)	(156,611,000)
Unrealized Gain(Loss)	1,161,000	(246,000)
Fair Value	45,507,000	38,137,000
Other | External Trust Funds
External and Internal Trust Funds:
Cost		115,000	$ 141,000
Purchases	544,000	370,000
Net Proceeds	(1,361,000)	(1,621,000)
Unrealized Gain(Loss)	0	0
Fair Value	$ (702,000)	$ (1,110,000)