Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurements
Transfer from Level 1 to Level 2, assets	$ 0	$ 0
Transfer from Level 2 to Level 1, assets	0	0
Overnight repurchase agreements	24,000
Recurring \| Fair value
Fair Value Measurements
Assets fair value	342,660	186,443
Recurring \| Money market funds and overnight repurchase agreements
Fair Value Measurements
Assets fair value	24,000
Recurring \| Money market funds and overnight repurchase agreements \| Fair value
Fair Value Measurements
Assets fair value	54,077
Recurring \| Money market funds and overnight repurchase agreements \| Fair value
Fair Value Measurements
Assets fair value		55,283
Recurring \| Corporate debt securities \| Fair value
Fair Value Measurements
Assets fair value		980
Recurring \| U.S. government-sponsored enterprise securities:
Fair Value Measurements
Assets fair value	24,290
Recurring \| U.S. government-sponsored enterprise securities: \| Fair value
Fair Value Measurements
Assets fair value	24,290
Recurring \| Corporate debt securities \| Fair value
Fair Value Measurements
Assets fair value	73,651	70,668
Recurring \| Commercial paper obligations \| Fair value
Fair Value Measurements
Assets fair value	125,805	15,250
Recurring \| Foreign government bonds \| Fair value
Fair Value Measurements
Assets fair value		18,520
Recurring \| Asset-backed securities \| Fair value
Fair Value Measurements
Assets fair value	64,837	25,742
Recurring \| Quoted Prices in Active Markets (Level 1)
Fair Value Measurements
Assets fair value	30,077	55,283
Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds and overnight repurchase agreements
Fair Value Measurements
Assets fair value	30,077
Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds and overnight repurchase agreements
Fair Value Measurements
Assets fair value		55,283
Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Measurements
Assets fair value	312,583	131,160
Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Assets fair value		980
Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Assets fair value	73,651	70,668
Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	125,805	15,250
Recurring \| Significant Other Observable Inputs (Level 2) \| Foreign government bonds
Fair Value Measurements
Assets fair value		18,520
Recurring \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair Value Measurements
Assets fair value	$ 64,837	$ 25,742

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=36462937&loc=d3e19207-110258

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+ References

No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: 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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