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Marketable Securities and Fair Value Measurements - Company's Assets Measured at Fair Value (Detail) - USD ($)
Jun. 30, 2023
Sep. 30, 2022
Investment Holdings [Line Items]    
Total $ 88,413,000 $ 89,300,000
Equities [Member]    
Investment Holdings [Line Items]    
Total   12,149,000
Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total   5,337,000
Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 3,345,000 4,794,000
Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total 34,635,000 37,339,000
Government Securities [Member]    
Investment Holdings [Line Items]    
Total 50,331,000 29,327,000
Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total 102,000 354,000
Level 1 [Member]    
Investment Holdings [Line Items]    
Total 53,778,000 51,961,000
Level 1 [Member] | Equities [Member]    
Investment Holdings [Line Items]    
Total   12,149,000
Level 1 [Member] | Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total   5,337,000
Level 1 [Member] | Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 3,345,000 4,794,000
Level 1 [Member] | Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 1 [Member] | Government Securities [Member]    
Investment Holdings [Line Items]    
Total 50,331,000 29,327,000
Level 1 [Member] | Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total 102,000 354,000
Level 2 [Member]    
Investment Holdings [Line Items]    
Total 34,635,000 37,339,000
Level 2 [Member] | Equities [Member]    
Investment Holdings [Line Items]    
Total   0
Level 2 [Member] | Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total   0
Level 2 [Member] | Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 2 [Member] | Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total 34,635,000 37,339,000
Level 2 [Member] | Government Securities [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 2 [Member] | Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 3 [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 3 [Member] | Equities [Member]    
Investment Holdings [Line Items]    
Total   0
Level 3 [Member] | Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total   0
Level 3 [Member] | Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 3 [Member] | Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 3 [Member] | Government Securities [Member]    
Investment Holdings [Line Items]    
Total 0 0
Level 3 [Member] | Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total $ 0 $ 0