Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018		Jun. 30, 2018	Dec. 31, 2017
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 89,626	$ 78,768	[1]	$ 91,751	$ 75,383
Cash and cash equivalents, Fair Value	89,626	78,768
Short-term investments, Amortized Cost	34,299	89,359
Short-term investments, Gross Unrealized Gains	16
Short-term investments, Gross Unrealized Losses		(38)
Short-term investments, Fair Value	34,315	89,321
Cash equivalents and short-term investments, Amortized Cost	123,925	168,127
Cash equivalents and short-term investments, Gross Unrealized Gains	16
Cash equivalents and short-term investments, Gross Unrealized Losses		(38)
Cash equivalents and short-term investments, Fair Value	123,941	168,089
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	4,455	3,733
Cash and cash equivalents, Fair Value	4,455	3,733
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	85,171	73,238
Cash and cash equivalents, Fair Value	85,171	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 Notes [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	2,291
Short-term investments, Gross Unrealized Gains	2
Short-term investments, Fair Value	2,293
U.S. Treasury Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	2,293	3,996
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Fair Value	2,294	3,996
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	7,745	34,611
Short-term investments, Gross Unrealized Gains	3
Short-term investments, Gross Unrealized Losses		(21)
Short-term investments, Fair Value	7,748	34,590
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	17,674	41,371
Short-term investments, Gross Unrealized Gains	5
Short-term investments, Gross Unrealized Losses		(14)
Short-term investments, Fair Value	17,679	41,357
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	4,296	9,381
Short-term investments, Gross Unrealized Gains	5
Short-term investments, Gross Unrealized Losses		(3)
Short-term investments, Fair Value	$ 4,301	$ 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.