Fair Value Of Financial Instruments - Summary of Carrying Values and the Estimated Fair Values of the Debt Financial Instruments (Detail) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Carrying Value [Member]
Schedule of carrying values and the estimated fair values of the debt financial instruments [Line Items]
Term Loan Agreement due May 5, 2024	$ 467,182	$ 467,147
Estimated Fair Value [Member]
Schedule of carrying values and the estimated fair values of the debt financial instruments [Line Items]
Term Loan Agreement due May 5, 2024	$ 467,182	$ 465,185