Investments - Amortized Cost, Unrealized Holding Gains and (Losses) and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 31,501	$ 41,403
Unrealized Losses	(48)	(51)
Fair Value	31,453	41,352
Short-term commercial paper and corporate bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	31,501	41,403
Unrealized Losses	(48)	(51)
Fair Value	$ 31,453	$ 41,352

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant