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Derivatives - Additional Information (Detail)
12 Months Ended
Sep. 30, 2020
USD ($)
Derivative
Sep. 30, 2019
USD ($)
Derivative
Derivative [Line Items]    
Significant concentration of credit risk associated with our derivative instruments $ 0 $ 0
Cross Currency Swaps [Member]    
Derivative [Line Items]    
Fair value of swaps, net asset   $ 1,000,000
Fair value of swaps, net liability $ 1,000,000  
Fair Value Hedging [Member] | Interest Rate Swap-Fixed to Variable [Member] | Interest Rate Risk [Member]    
Derivative [Line Items]    
Derivatives held to manage interest rate risk | Derivative 0 0
Net Investment Hedge [Member] | Cross Currency Swaps [Member]    
Derivative [Line Items]    
Notional Amount $ 250,000,000  
Net cash interest received 4,000,000 $ 4,000,000
Cumulative gain (loss) related to swaps $ 2,000,000 $ 5,000,000
Net Investment Hedge [Member] | Cross Currency Swaps [Member] | Fixed Rate Debt [Member]    
Derivative [Line Items]    
Derivative Instruments Frequency of Cash Settlements semi-annually