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v3.19.3.a.u2

Schedule II - Valuation and Qualifying Accounts (Tables) | 12 Months Ended
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Dec. 31, 2019
Valuation and Qualifying Accounts |
Valuation and qualifying accounts | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Additions - |  | Additions - |  | ​ |  | ​ | ​
​ | ​ | Balance at | ​ | Charged to | ​ | Charged to | ​ | ​ | ​ | ​ | Balance at
​ | ​ | Beginning of | ​ | Costs and | ​ | Other Accounts - | ​ | Deductions - | ​ | End of
Description | ​ | Period | ​ | Expenses | ​ | Describe | ​ | Describe | ​ | Period
Allowance for doubtful accounts: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  | ​ | ​ |
For the year ended December 31, 2019 | ​ | $ | 13,238 | ​ | $ | 9,461 | ​ | $ | — | ​ | $ | 8,282 | (1) | ​ | $ | 14,417
For the year ended December 31, 2018 | ​ |  | 12,717 | ​ |  | 9,475 | ​ |  | — | ​ |  | 8,954 | (1) | ​ |  | 13,238
For the year ended December 31, 2017 | ​ | $ | 12,040 | ​ | $ | 8,598 | ​ | $ | — | ​ | $ | 7,921 | (1) | ​ | $ | 12,717
​
(1) | Uncollectable accounts written off.

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- Definition Tabular disclosure of allowance for credit loss of financing receivable, classified as current. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447 + Details
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