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Accounts receivable reserves - Schedule of Accounts Receivable Reserve (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Changes in the reserves against current billed accounts receivable |  |  |
Contractual retainage provisions | $ 13,400 | $ 10,300 |
Accounts receivable, billed and unbilled |  |  |
Changes in the reserves against current billed accounts receivable |  |  |
Balance at beginning of year | 6,843 | 4,226 | $ 3,385
Additions to reserve | 243 | 5,106 | 2,335
Deductions | (2,801) | (2,489) | (1,494)
Balance at end of year | $ 4,285 | $ 6,843 | $ 4,226

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