Fair Value Measurements and Marketable Securities - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 90,768
Gross Unrealized Gains	2
Gross Unrealized Losses	(25)
Fair Value	90,745
U.S. Government Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	38,771
Gross Unrealized Gains	1
Gross Unrealized Losses	(11)
Fair Value	38,761
U.S. Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	35,607
Gross Unrealized Gains	1
Gross Unrealized Losses	(14)
Fair Value	35,594
U.S. Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	16,390
Fair Value	$ 16,390

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

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- Definition

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.

+ References

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+ References

No definition available.

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