Marketable Securities - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Market Value	$ 249,858	$ 299,849
U.S. Treasury Bills (due within one year) [Member]
Marketable Securities [Line Items]
Amortized Cost	249,842	299,813
Unrealized Gain	16	36
Market Value	$ 249,858	$ 299,849

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562

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- Definition

Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

No definition available.

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- Definition

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.

+ References

No definition available.

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