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Marketable Securities - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Marketable Securities [Abstract]        
Net unrealized gains $ 311,000 $ 647,000 $ 465,000 $ 1,590,000
Transfers of investments between Level 1 and Level 2 $ 0 $ 0 $ 0 $ 0