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Accounts receivable reserves | 12 Months Ended
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Sep. 30, 2018
Receivables [Abstract] |
Accounts receivable reserves | Accounts receivable reserves Changes in the reserves against accounts receivable were as follows (in thousands):
| Year ended September 30,
| 2018 |  | 2017 |  | 2016
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 |
In evaluating the net realizable value of accounts receivable, we consider such factors as current economic trends, customer credit-worthiness, and changes in the customer payment terms and collection trends. Changes in the assumptions used in analyzing a specific account receivable may result in a reserve being recognized in the period in which the change occurs. At September 30, 2018 and 2017, $13.4 million and $10.3 million of our unbilled receivables related to amounts pursuant to contractual retainage provisions. We anticipate that the majority of the fiscal 2018 balance will be billed and collected during fiscal year 2019.

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- Definition The entire disclosure for valuation and qualifying accounts and reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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