Fair Value Measurements (Available-For-Sale Marketable Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Marketable securities classified as available-for-sale
Cost or Amortized Cost	$ 75,970	$ 54,566
Gross Unrealized Gains	4	2,021
Gross Unrealized Losses	(32)	(2)
Total Estimated Fair Value	75,942	56,585
Commercial paper
Marketable securities classified as available-for-sale
Cost or Amortized Cost	39,637	32,350
Gross Unrealized Gains	4	71
Gross Unrealized Losses	(9)
Total Estimated Fair Value	39,632	32,421
Corporate debt securities
Marketable securities classified as available-for-sale
Cost or Amortized Cost	36,333	14,868
Gross Unrealized Gains		3
Gross Unrealized Losses	(23)	(2)
Total Estimated Fair Value	$ 36,310	14,869
Corporate equity securities
Marketable securities classified as available-for-sale
Cost or Amortized Cost		7,348
Gross Unrealized Gains		1,947
Total Estimated Fair Value		$ 9,295

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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