Fair Value Measurements - Recurring Basis (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Financial Assets
Marketable securities	$ 329,139	$ 185,323
Transfers between levels \| item	0	0
U.S. Treasuries
Financial Assets
Marketable securities	$ 123,559
Government agency securities
Financial Assets
Marketable securities	58,950
Money market funds
Financial Assets
Marketable securities	134,635	$ 185,323
U.S. Treasuries
Financial Assets
Marketable securities	11,995
Recurring
Financial Assets
Cash and cash equivalents		185,323
Total financial assets	330,758	186,942
Recurring \| U.S. Treasuries
Financial Assets
Marketable securities	123,559
Recurring \| Government agency securities
Financial Assets
Marketable securities	58,950
Recurring \| Money market funds
Financial Assets
Cash and cash equivalents	134,635
Restricted cash	1,619	1,619
Recurring \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	11,995
Recurring \| Level 1
Financial Assets
Cash and cash equivalents		185,323
Total financial assets	330,758	186,942
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Marketable securities	123,559
Recurring \| Level 1 \| Government agency securities
Financial Assets
Marketable securities	58,950
Recurring \| Level 1 \| Money market funds
Financial Assets
Cash and cash equivalents	134,635
Restricted cash	1,619	$ 1,619
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	$ 11,995

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- Definition

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

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- References

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- Definition

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

+ 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 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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- Definition

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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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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