Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Sep. 30, 2018
Assets:
Marketable securities			$ 18,059
Cash deposits		$ 61,500	71,600
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	[1]	18,417	18,413
Marketable securities	[2]	17,210	18,059
Total		35,627	36,472
Fair Value, Measurements, Recurring | Active Markets for Identical Instruments (Level 1)
Assets:
Cash equivalents	[1]	18,417	18,413
Marketable securities	[2]	17,210	18,059
Total		$ 35,627	$ 36,472

[1] Included in cash and cash equivalents on our condensed consolidated balance sheet at December 31, 2018 and September 30, 2018. Not included in these tables are cash deposits of $61.5 million and $71.6 million at December 31, 2018 and September 30, 2018, 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 sheet at December 31, 2018 and September 30, 2018.