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Fair Value Measurements - Carrying Value and Estimated Fair Value of Long-term Debt, Including Current Maturities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, including current maturities, carrying value $ 994,102 $ 956,918
Long-term debt, including current maturities, fair value $ 993,400 $ 975,000