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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, 2018	Sep. 30, 2017
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items]
Exercisable stock options, included in the diluted EPS calculation					367,000	463,000
Effect of dilutive stock options					231,000	284,000
Anti-dilutive exercisable stock options, not included in the diluted EPS calculation					0	0
Net unrealized gains (losses)					$ 612,000	$ 1,183,000
Transfers of investments between Level 1 and Level 2			$ 0		0	0
Payments to acquire marketable securities					256,124,000	492,674,000
Impairment loss recognized on an asset group					$ 0	0
Time for the collection of cost associated with Pre- contract					1 year
Valuation allowance			0		$ 0	$ 0
U.S. corporate income tax rate	21.00%	35.00%			24.30%	34.00%
Tax benefit recorded due to re-measurement of deferred tax liability			100,000	$ 700,000
Net revenues					$ 98,614,000	$ 80,608,000
Total long-term assets			$ 7,942,000		$ 7,942,000	$ 5,775,000
Entities Owned by One Global Company [Member] | Revenue [Member]
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration					3.00%	13.00%
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					0.00%	10.00%
Three to Four Years Old Inventory [Member]
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items]
Cost basis reduction in inventory, percentage					50.00%
Inventory, minimum time period on the shelf, years					3 years
Inventory, maximum time period on the shelf, years					4 years
Four to Five Years Old Inventory [Member]
Nature Of Operations And Summary Of Significant Accounting Policies [Line Items]
Cost basis reduction in inventory, percentage					75.00%
Inventory, minimum time period on the shelf, years					4 years
Inventory, maximum time period on the shelf, years					5 years
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

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