Financial Instruments (Details) - Measured on a recurring basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Measurements
Adjusted Cost, assets	$ 71,813	$ 93,239
Fair Value, assets	71,818	93,189
Unrealized Gains, assets	10	1
Unrealized Losses, assets	(5)	(51)
Cash and Cash Equivalents
Fair Value Measurements
Fair Value, assets	19,828	40,395
Short-term investments
Fair Value Measurements
Fair Value, assets	48,789	52,794
Long-term investments
Fair Value Measurements
Fair Value, assets	3,201
Cash
Fair Value Measurements
Fair value, cash and cash equivalents	14,782	30,501
Adjusted Cost, assets	14,782	30,501
Cash \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	14,782	30,501
Level 1
Fair Value Measurements
Adjusted Cost, assets	20,129	17,323
Fair Value, assets	20,130	17,309
Unrealized Gains, assets	1
Unrealized Losses, assets		(14)
Level 1 \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, assets	5,046	6,903
Level 1 \| Short-term investments
Fair Value Measurements
Fair Value, assets	15,084	10,406
Level 1 \| Money market funds
Fair Value Measurements
Fair value, cash and cash equivalents	5,046	6,903
Adjusted Cost, assets	5,046	6,903
Level 1 \| Money market funds \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	5,046	6,903
Level 1 \| U.S. government notes
Fair Value Measurements
Adjusted Cost, available-for-sale securities	15,083	10,420
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities		(14)
Fair Value, available-for-sale investments	15,084	10,406
Level 1 \| U.S. government notes \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	15,084	10,406
Level 2
Fair Value Measurements
Adjusted Cost, available-for-sale securities	36,902	45,415
Unrealized Gains, available-for-sale securities	9	1
Unrealized Losses, available-for-sale securities	(5)	(37)
Fair Value, available-for-sale investments	36,906	45,379
Level 2 \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments		2,991
Level 2 \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	33,705	42,388
Level 2 \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	3,201
Level 2 \| Corporate bonds
Fair Value Measurements
Adjusted Cost, available-for-sale securities	22,098	21,612
Unrealized Gains, available-for-sale securities	9	1
Unrealized Losses, available-for-sale securities	(5)	(37)
Fair Value, available-for-sale investments	22,102	21,576
Level 2 \| Corporate bonds \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	18,901	21,576
Level 2 \| Corporate bonds \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	3,201
Level 2 \| Commercial paper
Fair Value Measurements
Adjusted Cost, available-for-sale securities	14,804	23,803
Fair Value, available-for-sale investments	14,804	23,803
Level 2 \| Commercial paper \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments		2,991
Level 2 \| Commercial paper \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	$ 14,804	$ 20,812

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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.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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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