Fair Value of Financial Assets and Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value of Financial Assets and Liability
Carrying amount of cash and cash equivalents and short term investments	$ 272,386	$ 148,082
Fair value of financial assets and liability
Marketable Securities:	214,139	113,846
Corporate Debt Securities
Fair value of financial assets and liability
Marketable Securities:	214,139	96,901
Government Funds
Fair value of financial assets and liability
Marketable Securities:		16,945
Level 1 \| Money Market Funds and Government Funds in Aggregate
Fair value of financial assets and liability
Fair value of assets	57,856	34,698
Recurring \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	271,995	146,703
Recurring \| Corporate Debt Securities \| Estimated fair value
Fair value of financial assets and liability
Marketable Securities:	214,139	96,901
Recurring \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	57,856	17,753
Recurring \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		15,104
Marketable Securities:		16,945
Recurring \| Level 1 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	57,856	34,698
Recurring \| Level 1 \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	57,856	17,753
Recurring \| Level 1 \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Marketable Securities:		16,945
Recurring \| Level 2 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	214,139	112,005
Recurring \| Level 2 \| Corporate Debt Securities \| Estimated fair value
Fair value of financial assets and liability
Marketable Securities:	$ 214,139	96,901
Recurring \| Level 2 \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		$ 15,104

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means 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. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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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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