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Accounts Receivable (Tables) | 12 Months Ended
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Dec. 31, 2019
Accounts Receivable |
Schedule of components of accounts receivables | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
December 31, |  | 2019 |  | 2018
(in thousands) | ​ |  | ​ | ​ |  | ​
Trade receivables: |  | ​ |  |  | ​ |
Billed | ​ | $ | 191,579 | ​ | $ | 266,660
Unbilled | ​ |  | 52,052 | ​ |  | 56,408
Other receivables | ​ |  | 4,124 | ​ |  | 5,278
Total | ​ |  | 247,755 | ​ |  | 328,346
Less: allowance for doubtful accounts | ​ |  | (5,181) | ​ |  | (4,813)
Accounts receivable, net | ​ | $ | 242,574 | ​ | $ | 323,533
Schedule of changes in allowance for doubtful accounts | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
Years Ended December 31, |  | 2019 |  | 2018
(in thousands) | ​ |  | ​ | ​ |  | ​
Beginning balance | ​ | $ | 4,813 | ​ | $ | 4,471
Bad debt expense | ​ |  | 1,481 | ​ |  | 588
Accounts written-off | ​ |  | (1,142) | ​ |  | (260)
Recoveries | ​ |  | 29 | ​ |  | 14
Ending balance | ​ | $ | 5,181 | ​ | $ | 4,813

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