Fair Value Measurements - Summary of the Levels of the Fair Value Hierarchy into Which the Company's Financial Assets Fall (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Measurements
Total financial assets	$ 45,893	$ 282,045
U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents		110,529
Investments in securities	9,273	79,395
Government Debt Securities
Fair Value Measurements
Investments in securities		1,105
Level 1
Fair Value Measurements
Total financial assets	9,464	108,627
Level 1 \| U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents		100,500
Investments in securities	9,273	7,977
Level 2
Fair Value Measurements
Total financial assets	36,429	173,418
Level 2 \| U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents		10,029
Investments in securities		71,418
Level 2 \| Government Debt Securities
Fair Value Measurements
Investments in securities		1,105
Government Securities Money Market
Fair Value Measurements
Cash and cash equivalents	36,429	90,866
Government Securities Money Market \| Level 2
Fair Value Measurements
Cash and cash equivalents	36,429	90,866
Common Stock
Fair Value Measurements
Investments in securities	191	150
Common Stock \| Level 1
Fair Value Measurements
Investments in securities	$ 191	$ 150

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
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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