Cash and Cash Equivalents, Restricted Cash and Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and cash equivalents:
Amortized Cost	$ 199,798	$ 11,769
Gross Unrealized Gains	49	2
Gross Unrealized Losses	(8)	(18)
Estimated Fair Value	199,839	11,753
Level 2 \| U.S. treasuries
Cash and cash equivalents:
Amortized Cost	177,679	2,249
Gross Unrealized Gains	30	0
Gross Unrealized Losses	(3)	(3)
Estimated Fair Value	177,706	2,246
Level 2 \| Fixed-income debt securities
Cash and cash equivalents:
Amortized Cost	15,679	9,520
Gross Unrealized Gains	19	2
Gross Unrealized Losses	(5)	(15)
Estimated Fair Value	15,693	$ 9,507
Level 2 \| Commercial paper
Cash and cash equivalents:
Amortized Cost	6,440
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	$ 6,440

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ 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 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ 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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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