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Fair Value Measurements - Estimated Fair Value of Long-term Debt, Including Current Maturities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2021
Sep. 30, 2020
Fair Value Disclosures [Abstract]    
Long-term debt and financing leases, including current maturities, carrying value $ 725,433 $ 743,250
Long-term debt and financing leases, including current maturities, fair value $ 731,400 $ 768,700