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.