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Marketable Securities - Company's Assets Measured at Fair Value (Detail) - USD ($)
Dec. 31, 2017
Sep. 30, 2017
Investment Holdings [Line Items]    
Total $ 88,340,000 $ 87,886,000
Equities [Member]    
Investment Holdings [Line Items]    
Total 10,777,000 11,338,000
Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total 8,730,000 7,155,000
Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 4,508,000 3,417,000
Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total 16,387,000 7,196,000
Government Securities [Member]    
Investment Holdings [Line Items]    
Total 42,499,000 54,542,000
Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total 5,439,000 4,238,000
Level 1 [Member]    
Investment Holdings [Line Items]    
Total 71,953,000 80,690,000
Level 1 [Member] | Equities [Member]    
Investment Holdings [Line Items]    
Total 10,777,000 11,338,000
Level 1 [Member] | Mutual Funds [Member]    
Investment Holdings [Line Items]    
Total 8,730,000 7,155,000
Level 1 [Member] | Exchange Traded Funds [Member]    
Investment Holdings [Line Items]    
Total 4,508,000 3,417,000
Level 1 [Member] | Government Securities [Member]    
Investment Holdings [Line Items]    
Total 42,499,000 54,542,000
Level 1 [Member] | Cash and Money Funds [Member]    
Investment Holdings [Line Items]    
Total 5,439,000 4,238,000
Level 2 [Member]    
Investment Holdings [Line Items]    
Total 16,387,000 7,196,000
Level 2 [Member] | Corporate Bonds [Member]    
Investment Holdings [Line Items]    
Total $ 16,387,000 $ 7,196,000