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Cash, Cash Equivalents and Short-term Investments - Narrative (Details) $ in Thousands	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Debt securities in an unrealized loss position for less than 12 months | security	5	10	21
Aggregate fair value of securities in an unrealized loss position	$ 10,400	$ 22,900	$ 46,600
Securities in unrealized loss positions, greater than one year | security		1
Aggregate fair value of securities in an unrealized loss position longer than 12 months		$ 5,000
Securities held	53,637	67,230	114,448
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Securities held	$ 7,900	$ 12,000	$ 11,500

X

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 7
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