Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Sep. 30, 2020
Assets:
Marketable securities		$ 30,437	$ 25,513
Cash deposits		197,600	122,100
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	[1]	194	35,275
Marketable securities	[2]	30,437	25,513
Total		30,631	60,788
Fair Value, Measurements, Recurring | Active Markets for Identical Instruments (Level 1)
Assets:
Cash equivalents	[1]	194	35,275
Marketable securities	[2]	30,437	25,513
Total		$ 30,631	$ 60,788

[1] Included in cash and cash equivalents on our condensed consolidated balance sheets at March 31, 2021 and September 30, 2020. Not included in these tables are cash deposits of $197.6 million and $122.1 million at March 31, 2021 and September 30, 2020, respectively.

[2] Represents securities held under a supplemental retirement and savings plan for senior management employees, which are distributed upon termination or retirement of the employees. Included in marketable securities on our condensed consolidated balance sheets at March 31, 2021 and September 30, 2020.