Financial Instruments (Details) - Measured on a recurring basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Measurements
Adjusted Cost, assets	$ 91,067	$ 85,918
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(66)	(62)
Fair Value, assets	91,002	85,856
Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	35,373	29,761
Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	54,544	44,057
Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,085	12,038
Cash
Fair Value Measurements
Fair value, cash and cash equivalents	28,687	24,367
Cash \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	28,687	24,367
Level 1
Fair Value Measurements
Adjusted Cost, assets	13,997	14,454
Unrealized Losses, available-for-sale securities	(29)	(13)
Fair Value, assets	13,968	14,441
Level 1 \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	5,039	5,394
Level 1 \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	8,929	4,098
Level 1 \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments		4,949
Level 1 \| Money market funds
Fair Value Measurements
Fair value, cash and cash equivalents	5,039	5,394
Level 1 \| Money market funds \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	5,039	5,394
Level 1 \| U.S. government notes
Fair Value Measurements
Adjusted Cost, available-for-sale securities	8,958	9,060
Unrealized Losses, available-for-sale securities	(29)	(13)
Fair Value, available-for-sale investments	8,929	9,047
Level 1 \| U.S. government notes \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	8,929	4,098
Level 1 \| U.S. government notes \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments		4,949
Level 2 \| Available-for-sale investments
Fair Value Measurements
Adjusted Cost, available-for-sale securities	48,383	47,097
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(37)	(49)
Fair Value, available-for-sale investments	48,347	47,048
Level 2 \| Available-for-sale investments \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	1,647
Level 2 \| Available-for-sale investments \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	45,615	39,959
Level 2 \| Available-for-sale investments \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,085	7,089
Level 2 \| Corporate bonds
Fair Value Measurements
Adjusted Cost, available-for-sale securities	25,755	24,943
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(37)	(49)
Fair Value, available-for-sale investments	25,719	24,894
Level 2 \| Corporate bonds \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	24,634	17,805
Level 2 \| Corporate bonds \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,085	7,089
Level 2 \| Commercial paper
Fair Value Measurements
Adjusted Cost, available-for-sale securities	22,628	22,154
Fair Value, available-for-sale investments	22,628	22,154
Level 2 \| Commercial paper \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	1,647
Level 2 \| Commercial paper \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	$ 20,981	$ 22,154

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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.

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-Publisher FASB
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