Quarterly financial data (unaudited) (Details) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Operating revenues	$ 338,761	$ 366,239	$ 330,944	$ 341,674	$ 332,209	$ 382,623	$ 358,860	$ 356,600
Operating margin	41,800	47,862	65,532	62,515	40,256	59,209	51,712	66,032	$ 217,709	$ 217,209	$ 224,976
Net margin	$ (1,942)	$ 8,471	$ 26,162	$ 23,204	$ 3,196	$ 18,286	$ 9,383	$ 23,596	$ 55,895	$ 54,461	$ 51,199
Targeted margins for interest ratio									1.14	1.14	1.14
Maximum
Targeted margins for interest ratio	1.14

Represents the ratio of targeted margins for interest.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

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Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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