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Nature of Operations and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017 | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2019 | Sep. 30, 2018
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Exercisable stock options, included in the diluted EPS calculation |  |  |  |  |  | 307,000 | 367,000 |
Effect of dilutive stock options |  |  |  |  |  | 157,000 | 231,000 |
Anti-dilutive exercisable stock options, not included in the diluted EPS calculation |  |  |  |  |  | 0 | 0 |
Net unrealized gains (losses) |  |  |  |  |  | $ 737,000 | $ 612,000 |
Transfers of investments between Level 1 and Level 2 |  |  | $ 0 |  |  | 0 | 0 |
Payments to acquire marketable securities |  |  |  |  |  | 188,066,000 | 256,124,000 | [1]
Impairment loss recognized on an asset group |  |  |  |  |  | 0 | 0 |
Valuation allowance |  |  | 0 |  |  | $ 0 | $ 0 |
U.S. corporate income tax rate | 21.00% | 35.00% |  |  |  | 21.00% | 24.30% |
Tax benefit recorded due to re-measurement of deferred tax liability |  |  | 100,000 |  | $ 700,000 |  |  |
Net revenue |  |  |  |  |  | $ 81,329,000 | $ 98,614,000 | [1]
Total long-term assets |  |  | 7,942,000 |  |  | 8,442,000 | 7,942,000 |
Costs and estimated earnings in excess of billings |  |  | $ 11,900,000 | [2] |  | 13,838,000 | $ 11,900,000 | [2]
Accounts receivable related to contracts with customers |  |  |  |  |  | 301,000 |  |
Current Liabilities [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Customer deposits related to contracts with customers |  |  |  |  |  | $ 1,918,000 |  |
Entities Owned by One Global Company [Member] | Revenue [Member] | Customer One [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Percentage of concentration |  |  |  |  |  | 10.00% | 10.00% |
Three to Four Years Old Inventory [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Inventory, minimum time period on the shelf, years |  |  |  |  |  | 3 years |  |
Inventory, maximum time period on the shelf, years |  |  |  |  |  | 4 years |  |
Cost basis reduction in inventory, percentage |  |  |  |  |  | 50.00% |  |
Four to Five Years Old Inventory [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Inventory, minimum time period on the shelf, years |  |  |  |  |  | 4 years |  |
Inventory, maximum time period on the shelf, years |  |  |  |  |  | 5 years |  |
Cost basis reduction in inventory, percentage |  |  |  |  |  | 75.00% |  |
Greater Than Five Years Old Inventory [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Inventory, minimum time period on the shelf, years |  |  |  |  |  | 5 years |  |
Inventory valuation estimate |  |  |  |  |  | $ 0 |  |
Government Securities [Member] |  |  |  |  |  |  |  |
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Payments to acquire marketable securities |  |  |  |  |  |  | $ 15,000,000 |
[1] | The amounts for the year ended September 30, 2018 have been adjusted to reflect the change in inventory accounting method, as described in Notes 1 and 2 to the Consolidated Financial Statements.
[2] | The amounts as of September 30, 2018 have been adjusted to reflect the change in inventory accounting method, as described in Notes 1 and 2 to the Consolidated Financial Statements.

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