Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018		Sep. 30, 2018	Dec. 31, 2017
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 92,673	$ 78,768	[1]	$ 75,015	$ 75,383
Cash and cash equivalents, Fair Value	92,673	78,768
Short-term investments, Amortized Cost	10,866	89,359
Short-term investments, Gross Unrealized Gains	4
Short-term investments, Gross Unrealized Losses		(38)
Short-term investments, Fair Value	10,870	89,321
Cash equivalents and short-term investments, Amortized Cost	103,539	168,127
Cash equivalents and short-term investments, Gross Unrealized Gains	4
Cash equivalents and short-term investments, Gross Unrealized Losses		(38)
Cash equivalents and short-term investments, Fair Value	103,543	168,089
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	3,595	3,733
Cash and cash equivalents, Fair Value	3,595	3,733
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	89,078	73,238
Cash and cash equivalents, Fair Value	89,078	73,238
Commercial Paper (Cash Equivalents) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost		1,797
Cash and cash equivalents, Fair Value		1,797
U.S. Treasury Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost		3,996
Short-term investments, Fair Value		3,996
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	3,324	34,611
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Gross Unrealized Losses		(21)
Short-term investments, Fair Value	3,325	34,590
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	6,234	41,371
Short-term investments, Gross Unrealized Gains	2
Short-term investments, Gross Unrealized Losses		(14)
Short-term investments, Fair Value	6,236	41,357
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	1,308	9,381
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Gross Unrealized Losses		(3)
Short-term investments, Fair Value	$ 1,309	$ 9,378

[1] Derived from the audited consolidated financial statements included in the Company’s Annual Report on Form 10K for the year ended December 31, 2018.