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Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 100,433	$ 329,997
Maturity period of short term interest-bearing securities included in cash and cash equivalents	Longer than 90 days
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 30,883	$ 55,200
Maturity period of short term interest-bearing securities included in cash and cash equivalents	90 days or less