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Revenue Recognition (Contract Balances) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Changes in contract liabilities    
Balance at Beginning of Period $ 2,746 $ 2,187
Additions 3,165 1,637
Deductions (2,170) (1,789)
Balance at End of Period 3,741 2,035
Contract assets 0 0
Revenue recognized included in contract liabilities at the beginning of the period $ 415 $ 467