Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018		Mar. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 92,036	$ 78,768	[1]	$ 67,745	$ 75,383
Cash and cash equivalents, Fair Value	92,036	78,768
Short-term investments, Amortized Cost	59,512	89,359
Short-term investments, Gross Unrealized Gains	12
Short-term investments, Gross Unrealized Losses		(38)
Short-term investments, Fair Value	59,524	89,321
Cash equivalents and short-term investments, Amortized Cost	151,548	168,127
Cash equivalents and short-term investments, Gross Unrealized Gains	12
Cash equivalents and short-term investments, Gross Unrealized Losses		(38)
Cash equivalents and short-term investments, Fair Value	151,560	168,089
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	7,019	3,733
Cash and cash equivalents, Fair Value	7,019	3,733
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	83,521	73,238
Cash and cash equivalents, Fair Value	83,521	73,238
Commercial Paper (Cash Equivalents) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	1,496	1,797
Cash and cash equivalents, Fair Value	1,496	1,797
U.S. Treasury Notes [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	2,283
Short-term investments, Fair Value	2,283
U.S. Treasury Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	2,279	3,996
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Fair Value	2,280	3,996
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	20,110	34,611
Short-term investments, Gross Unrealized Gains	2
Short-term investments, Gross Unrealized Losses		(21)
Short-term investments, Fair Value	20,112	34,590
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	30,559	41,371
Short-term investments, Gross Unrealized Gains	5
Short-term investments, Gross Unrealized Losses		(14)
Short-term investments, Fair Value	30,564	41,357
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	4,281	9,381
Short-term investments, Gross Unrealized Gains	4
Short-term investments, Gross Unrealized Losses		(3)
Short-term investments, Fair Value	$ 4,285	$ 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.