Investments and Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 5,004	$ 6,661
Short-term investments	71,854	73,338
Long-term investments	1,507	1,269
Total fair value of financial assets	78,365	81,268
Fair Value, Measurements, Recurring \| Level 1 Inputs
Assets:
Cash equivalents	5,004	6,661
Short-term investments	0	0
Long-term investments	0	0
Total fair value of financial assets	5,004	6,661
Fair Value, Measurements, Recurring \| Level 2 Inputs
Assets:
Cash equivalents	0	0
Short-term investments	71,854	73,338
Long-term investments	1,507	1,269
Total fair value of financial assets	73,361	74,607
Fair Value, Measurements, Recurring \| Level 3 Inputs
Assets:
Cash equivalents	0	0
Short-term investments	0	0
Long-term investments	0	0
Total fair value of financial assets	0	0
U.S. Treasury securities
Assets:
Short-term investments	8,838	8,101
U.S. Treasury securities \| Fair Value, Measurements, Recurring \| Level 1 Inputs
Assets:
Short-term investments	0	0
U.S. Treasury securities \| Fair Value, Measurements, Recurring \| Level 2 Inputs
Assets:
Short-term investments	8,838	8,101
U.S. Treasury securities \| Fair Value, Measurements, Recurring \| Level 3 Inputs
Assets:
Short-term investments	0	0
Corporate notes and bonds
Assets:
Short-term investments	63,016	65,237
Long-term investments	1,507	1,269
Corporate notes and bonds \| Fair Value, Measurements, Recurring \| Level 1 Inputs
Assets:
Short-term investments	0	0
Long-term investments	0	0
Corporate notes and bonds \| Fair Value, Measurements, Recurring \| Level 2 Inputs
Assets:
Short-term investments	63,016	65,237
Long-term investments	1,507	1,269
Corporate notes and bonds \| Fair Value, Measurements, Recurring \| Level 3 Inputs
Assets:
Short-term investments	0	0
Long-term investments	0	0
Money market securities
Assets:
Cash equivalents	5,004	6,661
Money market securities \| Fair Value, Measurements, Recurring \| Level 1 Inputs
Assets:
Cash equivalents	5,004	6,661
Money market securities \| Fair Value, Measurements, Recurring \| Level 2 Inputs
Assets:
Cash equivalents	0	0
Money market securities \| Fair Value, Measurements, Recurring \| Level 3 Inputs
Assets:
Cash equivalents	0	$ 0
Corporate notes and bonds
Assets:
Cash equivalents	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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