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Inventory | 12 Months Ended
--- | ---
Dec. 31, 2021
Inventory |
Inventory | 6. Inventory ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31,
(in thousands) | ​ | 2021 |  | 2020
Parts and supplies | ​ | $ | 63,628 | ​ | $ | 57,433
Work in progress | ​ |  | 9,241 | ​ |  | 6,237
Inventory | ​ | $ | 72,869 | ​ | $ | 63,670
​ During the years ended December 31, 2021, 2020 and 2019, we recorded write-downs to inventory of $1.0 million, $1.3 million and $0.9 million, respectively, for inventory considered to be excess, obsolete or carried at an amount in excess of net realizable value.

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- Definition The entire disclosure for inventory. Includes, but is not limited to, the basis of stating inventory, the method of determining inventory cost, the classes of inventory, and the nature of the cost elements included in inventory. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -URI http://asc.fasb.org/topic&trid=2126998 + Details
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