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Description of the business and recent developments: Allowance for doubtful accounts (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:        
Bad debt expense, net of recoveries $ 0.8 $ 1.3 $ 1.6 $ 2.3