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Schedule II - Valuation and Qualifying Accounts
 12 Months Ended
Dec. 31, 2020
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]  
Schedule II - Valuation and Qualifying Accounts Financial Statement Schedules
Schedule II-Valuation and Qualifying Accounts (in thousands):
 Years Ended December 31, 2020, 2019, and 2018
 
Balance at
Beginning of
Year
Provision (Release) for Bad DebtsNet Write-offs
Balance at
End of Year
Accounts receivable allowance    
2020$56 $191 $(94)$153 
2019229 (79)(94)56 
2018259 142 (172)229 
 Years Ended December 31, 2020, 2019, and 2018
 
Balance at
Beginning of
Year
Provision for RefundsRefunds Issued
Balance at
End of Year
Refund reserve    
2020$554 $44,171 $(43,210)$1,515 
2019396 24,987 (24,829)554 
2018282 21,240 (21,126)396 
All other financial statement schedules are omitted because they are not applicable or the information is included in the Registrant’s consolidated financial statements or related notes.