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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, 2020
Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities $ 559,385 $ 69,370
Short Term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Maturity period of Available-for-sale securities longer than 90 days longer than 90 days
Available-for-sale securities $ 752  
Cash and Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Maturity period of Available-for-sale securities 90 days or less 90 days or less
Available-for-sale securities $ 558,633