Quarterly financial data (unaudited) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Revenue Recognition
Operating revenues	$ 409,583	$ 460,822	$ 358,127	$ 376,331	$ 338,761	$ 366,239	$ 330,944	$ 341,674
Operating margin	30,748	46,025	53,035	64,573	41,800	47,862	65,532	62,515	$ 194,381	$ 217,709	$ 217,209
Net margin	$ 9,771	$ 8,907	$ 13,145	$ 25,958	$ (1,942)	$ 8,471	$ 26,162	$ 23,204	$ 57,781	$ 55,895	$ 54,461
Targeted margins for interest ratio									1.14	1.14	1.14
Maximum
Revenue Recognition
Targeted margins for interest ratio					1.14