Marketable Securities and Fair Value Measurements - Company's Assets Measured at Fair Value (Detail) - USD ($)	Jun. 30, 2021	Sep. 30, 2020
Investment Holdings [Line Items]
Total	$ 95,345,000	$ 89,498,000
Equities [Member]
Investment Holdings [Line Items]
Total	14,679,000	11,949,000
Mutual Funds [Member]
Investment Holdings [Line Items]
Total	10,341,000	9,595,000
Exchange Traded Funds [Member]
Investment Holdings [Line Items]
Total	9,545,000	10,344,000
Corporate Bonds [Member]
Investment Holdings [Line Items]
Total	25,229,000	27,877,000
Government Securities [Member]
Investment Holdings [Line Items]
Total	30,998,000	16,147,000
Cash and Money Funds [Member]
Investment Holdings [Line Items]
Total	4,553,000	13,586,000
Level 1 [Member]
Investment Holdings [Line Items]
Total	70,116,000	61,621,000
Level 1 [Member] | Equities [Member]
Investment Holdings [Line Items]
Total	14,679,000	11,949,000
Level 1 [Member] | Mutual Funds [Member]
Investment Holdings [Line Items]
Total	10,341,000	9,595,000
Level 1 [Member] | Exchange Traded Funds [Member]
Investment Holdings [Line Items]
Total	9,545,000	10,344,000
Level 1 [Member] | Corporate Bonds [Member]
Investment Holdings [Line Items]
Total	0
Level 1 [Member] | Government Securities [Member]
Investment Holdings [Line Items]
Total	30,998,000	16,147,000
Level 1 [Member] | Cash and Money Funds [Member]
Investment Holdings [Line Items]
Total	4,553,000	13,586,000
Level 2 [Member]
Investment Holdings [Line Items]
Total	25,229,000	27,877,000
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
Level 2 [Member] | Corporate Bonds [Member]
Investment Holdings [Line Items]
Total	25,229,000	$ 27,877,000
Level 2 [Member] | Government Securities [Member]
Investment Holdings [Line Items]
Total	0
Level 2 [Member] | Cash and Money Funds [Member]
Investment Holdings [Line Items]
Total	0
Level 3 [Member]
Investment Holdings [Line Items]
Total	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
Level 3 [Member] | Corporate Bonds [Member]
Investment Holdings [Line Items]
Total	0
Level 3 [Member] | Government Securities [Member]
Investment Holdings [Line Items]
Total	0
Level 3 [Member] | Cash and Money Funds [Member]
Investment Holdings [Line Items]
Total	$ 0