Fair Value Measurements (Details) - Recurring basis - Quoted prices in active markets for identical assets (Level 1) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
U.S. Treasury money market funds
Fair Value Measurements
Investments in U.S. Treasury money market funds and U.S. Treasury securities with maturities less than 90 days	$ 33,904	$ 35,111
U.S. Treasury securities with maturities less than one year
Fair Value Measurements
Investments in U.S. Treasury securities	$ 412,320	$ 423,746

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

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=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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