Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Inputs, Level 3
Assets
Total assets	$ 0	$ 0
Liabilities
Total liabilities	0	0
Fair Value, Measurements, Recurring
Assets
Total assets	115,284,000	112,823,000
Liabilities
Cash-settled stock options	6,685,000
Total liabilities	6,685,000	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets
Total assets	2,530,000	1,156,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	112,754,000	111,667,000
Liabilities
Cash-settled stock options	6,685,000
Total liabilities	6,685,000
Money market funds \| Fair Value, Measurements, Recurring
Assets
Total assets	2,530,000	1,156,000
Money market funds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets
Total assets	2,530,000	1,156,000
U.S. Government agency bonds \| Fair Value, Measurements, Recurring
Assets
Total assets		1,982,000
U.S. Government agency bonds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets		1,982,000
U.S. Government bonds \| Fair Value, Measurements, Recurring
Assets
Total assets		1,297,000
U.S. Government bonds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets		1,297,000
Bank certificates of deposit \| Fair Value, Measurements, Recurring
Assets
Cash equivalents	1,201,000
Total assets	14,208,000	15,201,000
Bank certificates of deposit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	14,208,000	15,201,000
Commercial paper. \| Fair Value, Measurements, Recurring
Assets
Total assets	7,484,000	9,593,000
Commercial paper. \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	7,484,000	9,593,000
Corporate notes \| Fair Value, Measurements, Recurring
Assets
Cash equivalents		1,000,000
Total assets	65,638,000	61,752,000
Corporate notes \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	65,638,000	61,752,000
Asset-backed securities \| Fair Value, Measurements, Recurring
Assets
Total assets	25,424,000	21,842,000
Asset-backed securities \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	$ 25,424,000	$ 21,842,000

The fair value potion of the cash-settled stock options liability.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

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.

+ References

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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- References

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