Fair Value Measurements (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	$ 155,348,000	$ 170,029,000
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	0	0
Transfer of assets from Level 2 to Level 1 of the fair value measurement hierarchy	0	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Money market funds
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days	108,096,000	196,188,000
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| U.S. Treasury securities
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days		150,387,000
Investments in U.S. Treasury securities and Corporate debt securities	120,261,000
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Corporate debt securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	0	0
Recurring basis \| Significant other observable inputs (Level 2) \| Money market funds
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days	0	0
Recurring basis \| Significant other observable inputs (Level 2) \| U.S. Treasury securities
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days		0
Investments in U.S. Treasury securities and Corporate debt securities	0
Recurring basis \| Significant other observable inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	35,087,000	39,833,000
Recurring basis \| Significant unobservable inputs (Level 3) \| Money market funds
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days	0	0
Recurring basis \| Significant unobservable inputs (Level 3) \| U.S. Treasury securities
Fair Value Measurements
Investments in money market funds and U.S. Treasury securities with maturities less than 90 days		0
Investments in U.S. Treasury securities and Corporate debt securities	0
Recurring basis \| Significant unobservable inputs (Level 3) \| Corporate debt securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	$ 0	$ 0

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=65884222&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=73527792&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=66010992&loc=d3e26610-111562

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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
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 820
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-Section 50
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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.

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