Fair Value of Financial Instruments (Details) - Fair Value Measured on a Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets:
Cash and cash equivalents	$ 47,700	$ 108,000
Total Assets	251,070	250,832
Money Market Funds [Member]
Assets:
Cash and cash equivalents	47,713	62,353
U.S. Treasury Securities [Member]
Assets:
Available-for-sale securities	5,263	9,349
Government-sponsored Enterprises [Member]
Assets:
Available-for-sale securities	29,121	41,202
Corporate Debt Securities [Member]
Assets:
Available-for-sale securities	168,973	137,928
Quoted Prices in Active Markets for Identical Assets Level 1 [Member]
Assets:
Total Assets	47,713	62,353
Quoted Prices in Active Markets for Identical Assets Level 1 [Member] \| Money Market Funds [Member]
Assets:
Cash and cash equivalents	47,713	62,353
Quoted Prices in Active Markets for Identical Assets Level 1 [Member] \| U.S. Treasury Securities [Member]
Assets:
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets Level 1 [Member] \| Government-sponsored Enterprises [Member]
Assets:
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets Level 1 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets Level 2 [Member]
Assets:
Total Assets	203,357	188,479
Quoted Prices in Active Markets for Identical Assets Level 2 [Member] \| Money Market Funds [Member]
Assets:
Cash and cash equivalents	0	0
Quoted Prices in Active Markets for Identical Assets Level 2 [Member] \| U.S. Treasury Securities [Member]
Assets:
Available-for-sale securities	5,263	9,349
Quoted Prices in Active Markets for Identical Assets Level 2 [Member] \| Government-sponsored Enterprises [Member]
Assets:
Available-for-sale securities	29,121	41,202
Quoted Prices in Active Markets for Identical Assets Level 2 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale securities	168,973	137,928
Quoted Prices in Active Markets for Identical Assets Level 3 [Member]
Assets:
Total Assets	0	0
Quoted Prices in Active Markets for Identical Assets Level 3 [Member] \| Money Market Funds [Member]
Assets:
Cash and cash equivalents	0	0
Quoted Prices in Active Markets for Identical Assets Level 3 [Member] \| U.S. Treasury Securities [Member]
Assets:
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets Level 3 [Member] \| Government-sponsored Enterprises [Member]
Assets:
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets Level 3 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale securities	$ 0	$ 0

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

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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
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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