Investments - Amortized Cost and Fair Value on Available-for-Sale Investments and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale debt securities, Amortized Cost	$ 5,509
Available-for-sale debt securities, Gross Unrealized Losses	(16)
Available-for-sale debt securities, Fair Value	5,493
Amounts Maturing in One Year or Less
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale debt securities, Amortized Cost	74,408	$ 94,011
Available-for-sale debt securities, Gross Unrealized Gains	2
Available-for-sale debt securities, Gross Unrealized Losses	(20)	(110)
Available-for-sale debt securities, Fair Value	74,390	93,901
Amounts Maturing in One Year or Less \| Corporate Notes and Obligations
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale debt securities, Amortized Cost	27,069	38,044
Available-for-sale debt securities, Gross Unrealized Losses	(5)	(24)
Available-for-sale debt securities, Fair Value	27,064	38,020
Amounts Maturing in One Year or Less \| U.S. Government Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale debt securities, Amortized Cost	47,339	55,967
Available-for-sale debt securities, Gross Unrealized Gains	2
Available-for-sale debt securities, Gross Unrealized Losses	(15)	(86)
Available-for-sale debt securities, Fair Value	47,326	$ 55,881
Amounts Maturing in Greater than One Year \| Corporate Notes and Obligations
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale debt securities, Amortized Cost	5,509
Available-for-sale debt securities, Gross Unrealized Losses	(16)
Available-for-sale debt securities, Fair Value	$ 5,493

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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=82897076&loc=d3e27161-111563

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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