Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of assets measured at fair value on a recurring basis

Financial assets measured at fair value on a recurring basis at December 31, 2017 and 2016 are summarized as follows (in thousands):

Description	Balance as of December 31, 2017		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)
Assets:
Cash equivalents:
Money market funds	$	49,204	$	49,204	$	—	$	—
Overnight repurchase agreements	11,250		—		11,250		—
Marketable securities:
U.S. government-sponsored enterprise securities	18,181		—		18,181		—
Corporate debt securities	148,874		—		148,874		—
Certificates of deposit	7,794		—		7,794		—
Commercial paper obligations	108,630		—		108,630		—
Asset-backed securities	22,760		—		22,760		—
Total	$	366,693	$	49,204	$	317,489	$	—

Description	Balance as of December 31, 2016		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)
Assets:
Cash equivalents:
Money market funds	$	121,510	$	121,510	$	—	$	—
Overnight repurchase agreements	24,000		—		24,000		—
Marketable securities:
Corporate debt securities	47,906		—		47,906		—
Commercial paper obligations	84,436		—		84,436		—
Asset-backed securities	70,071		—		70,071		—
Total	$	347,923	$	121,510	$	226,413	$	—