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Supplemental balance sheet and cash flow information (trade accounts receivable, allowances for uncollectible accounts) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Accounts receivable, allowances for uncollectible accounts [Roll Forward]    
Balance, beginning of year $ 6,428 $ 4,985
Bad debt (benefit) expense (1,696) 3,374
Write-offs and other (1,704) (1,633)
Balance, end of period $ 3,028 $ 6,726