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Derivatives - Additional Information (Detail) - USD ($)
12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Derivative [Line Items]      
Significant concentration of credit risk associated with our derivative instruments $ 0 $ 0  
Maximum [Member]      
Derivative [Line Items]      
Fair value of derivative instruments not designated as hedges, liability 1,000,000 1,000,000  
Net Investment Hedge [Member] | Cross Currency Swaps [Member]      
Derivative [Line Items]      
Net cash interest received 4,000,000 4,000,000 $ 3,000,000
Fair value of swaps, net asset 12,000,000 29,000,000  
Cumulative gain related to swaps $ 15,000,000 $ 32,000,000  
Net Investment Hedge [Member] | Cross Currency Swaps [Member] | Fixed Rate Debt [Member]      
Derivative [Line Items]      
Derivative Instruments Frequency of Cash Settlements semi-annually