Cash, Cash Equivalents and Short-Term Investments - Summary of Estimated Value of Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	$ 53,314	$ 34,393	$ 91,216
Cash and cash equivalents, fair value	53,314	34,393
Short-term investments, amortized cost	13,367	27,156
Short-term investments, unrealized gain	5
Short-term investments, unrealized loss		(25)
Short-term investments, fair value	13,372	27,131
Cash
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	35,479	10,253
Cash and cash equivalents, fair value	35,479	10,253
Money Market Fund
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	5,835	140
Cash and cash equivalents, fair value	5,835	140
Reverse Repurchase Agreements
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	12,000	24,000
Cash and cash equivalents, fair value	12,000	24,000
U.S. Corporate Bonds
Cash And Cash Equivalents [Line Items]
Short-term investments, amortized cost	9,172	21,184
Short-term investments, unrealized gain	4
Short-term investments, unrealized loss		(19)
Short-term investments, fair value	9,176	21,165
International Corporate Bonds
Cash And Cash Equivalents [Line Items]
Short-term investments, amortized cost	4,195	5,972
Short-term investments, unrealized gain	1
Short-term investments, unrealized loss		(6)
Short-term investments, fair value	$ 4,196	$ 5,966