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Description of the business and recent developments: Gross receipts taxes, universal service fund and other surcharges (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Description of the business and recent developments:        
Excise taxes and surcharge $ 4.8 $ 3.3 $ 9.3 $ 7.0