Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents
Realized Investment Gains (Losses)	$ 0
Cash
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	36,112	$ 80,386
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	36,112	80,386
US Treasury Securities
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	235,463	73,983
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	94	0
Available-for-sale Securities, Gross Unrealized Losses	(10)	(9)
Available-for-sale Securities, Current	235,547	73,974
Corporate Debt Securities [Member]
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis		5,695
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments		0
Available-for-sale Securities, Gross Unrealized Losses		(3)
Available-for-sale Securities, Current		5,692
Short-term Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	235,463	79,678
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	94	0
Available-for-sale Securities, Gross Unrealized Losses	(10)	(12)
Available-for-sale Securities, Current	235,547	79,666
Cash and Cash Equivalents and Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	271,575	160,064
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	94	0
Available-for-sale Securities, Gross Unrealized Losses	(10)	(12)
Available-for-sale Securities, Current	$ 271,659	$ 160,052

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&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.

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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.

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

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