Financial Instruments (Details) - Measured on a recurring basis - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value Measurements
Adjusted Cost, assets	$ 64,797	$ 61,806
Unrealized Losses, available-for-sale securities	(18)	(15)
Fair Value, assets	64,779	61,791
Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	30,063	34,813
Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	32,707	22,970
Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	2,009	4,008
Cash
Fair Value Measurements
Fair value, cash and cash equivalents	22,467	24,708
Cash \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	22,467	24,708
Level 1
Fair Value Measurements
Adjusted Cost, assets	10,197	12,106
Unrealized Losses, available-for-sale securities	(9)	(1)
Fair Value, assets	10,188	12,105
Level 1 \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	5,098	10,105
Level 1 \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	5,090	2,000
Level 1 \| Money market funds
Fair Value Measurements
Fair value, cash and cash equivalents	5,098	10,105
Level 1 \| Money market funds \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	5,098	10,105
Level 1 \| U.S. government notes
Fair Value Measurements
Adjusted Cost, available-for-sale securities	5,099	2,001
Unrealized Losses, available-for-sale securities	(9)	(1)
Fair Value, available-for-sale investments	5,090	2,000
Level 1 \| U.S. government notes \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	5,090	2
Level 2 \| Available-for-sale investments
Fair Value Measurements
Adjusted Cost, available-for-sale securities	32,133	24,992
Unrealized Losses, available-for-sale securities	(9)	(14)
Fair Value, available-for-sale investments	32,124	24,978
Level 2 \| Available-for-sale investments \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	2,498
Level 2 \| Available-for-sale investments \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	27,617	20,970
Level 2 \| Available-for-sale investments \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	2,009	4,008
Level 2 \| Asset-backed securities
Fair Value Measurements
Adjusted Cost, available-for-sale securities	1,002	4,008
Unrealized Losses, available-for-sale securities	(1)
Fair Value, available-for-sale investments	1,001	4,008
Level 2 \| Asset-backed securities \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,001	4,008
Level 2 \| Corporate bonds
Fair Value Measurements
Adjusted Cost, available-for-sale securities	13,818	13,902
Unrealized Losses, available-for-sale securities	(8)	(14)
Fair Value, available-for-sale investments	13,810	13,888
Level 2 \| Corporate bonds \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	12,802	13,888
Level 2 \| Corporate bonds \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,008
Level 2 \| Commercial paper
Fair Value Measurements
Adjusted Cost, available-for-sale securities	14,815	7,082
Fair Value, available-for-sale investments	14,815	7,082
Level 2 \| Commercial paper \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	14,815	$ 7,082
Level 2 \| U.S. agency securities
Fair Value Measurements
Adjusted Cost, available-for-sale securities	2,498
Fair Value, available-for-sale investments	2,498
Level 2 \| U.S. agency securities \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	$ 2,498

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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