Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Assets Measured at Fair Value on Recurring Basis

Assets measured at fair value on a recurring basis were as follows:

Description	December 31, 2018		Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market mutual funds	$	31,740	$	31,740	$	—	$	—
Commercial paper		5,974		—		5,974		—
U.S. Treasury securities		35		35		—		—
Total cash equivalents	$	37,749	$	31,775	$	5,974	$	—
Short-term investments:
Asset-backed securities	$	19,520	$	—	$	19,520	$	—
Commercial paper		72,597		—		72,597		—
Corporate debt securities		54,461		—		54,461		—
U.S. Treasury securities		29,939		—		29,939		—
Total short-term investments	$	176,517	$	—	$	176,517	$	—
Total financial assets	$	214,266	$	31,775	$	182,491	$	—

Description	December 31, 2017		Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market mutual funds	$	44,182	$	44,182	$	—	$	—
Total cash equivalents	$	44,182	$	44,182	$	—	$	—
Short-term investments:
Asset-backed securities	$	7,422	$	—	$	7,422	$	—
Commercial paper		34,882		—		34,882		—
Corporate debt securities		26,856		—		26,856		—
U.S. Treasury securities		8,924		—		8,924		—
Total short-term investments	$	78,084	$	—	$	78,084	$	—
Total financial assets	$	122,266	$	44,182	$	78,084	$	—

Schedule of Reconciliation of Changes in the Fair Value of Financial Instruments

The reconciliations of changes in the fair value of financial instruments based on Level 3 inputs for the year ended December 31, 2017 consisted of:

(in thousands)
Fair value as of January 1, 2017	$	4,247
Change in fair value of convertible preferred    stock tranche liability		876
Reduction in tranche liability due to preferred    stock issuance		(5,123	)
Fair value as of December 31, 2017	$	—