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Marketable Securities - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Marketable Securities [Abstract]        
Net unrealized gains (losses) $ (577,000) $ (341,000) $ (2,012,000) $ 123,000
Transfers of investments between Level 1 and Level 2 $ 0 $ 0 $ 0 $ 0