Fair Value Measurements (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	$ 55,525,000	$ 155,348,000
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	0
Transfer of assets from Level 2 to Level 1 of the fair value measurement hierarchy	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Money market funds
Fair Value Measurements
Investments in money market funds, U.S. Treasury securities and corporate debt securities with maturities less than 90 days	136,634,000	108,096,000
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| U.S. Treasury securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	48,134,000	120,261,000
Fair Value, Measurements, Recurring \| 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
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Money market funds
Fair Value Measurements
Investments in money market funds, U.S. Treasury securities and corporate debt securities with maturities less than 90 days	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| U.S. Treasury securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	7,392,000	35,087,000
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Money market funds
Fair Value Measurements
Investments in money market funds, U.S. Treasury securities and corporate debt securities with maturities less than 90 days	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| U.S. Treasury securities
Fair Value Measurements
Investments in U.S. Treasury securities and Corporate debt securities	0	0
Fair Value, Measurements, Recurring \| 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=75030427&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=82897076&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=82896770&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.

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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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- Definition

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=66048111&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.

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