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Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents
Realized Investment Gains (Losses)	$ 7	$ 0	$ 0
Cash
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	61,661	46,600
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Current	61,661	46,600
US Treasury Securities
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	222,316	227,911
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	1
Available-for-sale Securities, Gross Unrealized Losses	(168)	(107)
Available-for-sale Securities, Current	222,148	227,805
Corporate Debt Securities [Member]
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	27,033	72,188
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	1
Available-for-sale Securities, Gross Unrealized Losses	(54)	(90)
Available-for-sale Securities, Current	26,979	72,099
Short-term Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	249,349	300,099
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	2
Available-for-sale Securities, Gross Unrealized Losses	(222)	(197)
Available-for-sale Securities, Current	249,127	299,904
Cash and Cash Equivalents and Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	311,010	346,699
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	2
Available-for-sale Securities, Gross Unrealized Losses	(222)	(197)
Available-for-sale Securities, Current	$ 310,788	$ 346,504

X

- Definition

Amount before tax of unrealized gain 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 (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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

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-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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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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-Name Accounting Standards Codification
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-Section 50
-Paragraph 1
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