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Investments - Marketable Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Amortized cost and fair value of marketable securities: |  |
Amortized Cost | $ 47,807 | $ 66,807
Gross Unrealized Losses | (27) | (20)
Estimated Fair Value | 47,780 | 66,787
U.S. treasuries |  |
Amortized cost and fair value of marketable securities: |  |
Amortized Cost | 33,914 | 40,013
Gross Unrealized Losses | (19) | (5)
Estimated Fair Value | 33,895 | 40,008
Government agency securities |  |
Amortized cost and fair value of marketable securities: |  |
Amortized Cost |  | 10,020
Gross Unrealized Losses |  | (8)
Estimated Fair Value |  | 10,012
Corporate debt |  |
Amortized cost and fair value of marketable securities: |  |
Amortized Cost | 10,894 | 13,780
Gross Unrealized Losses | (8) | (7)
Estimated Fair Value | 10,886 | 13,773
Commercial paper |  |
Amortized cost and fair value of marketable securities: |  |
Amortized Cost | 2,999 | 2,994
Estimated Fair Value | $ 2,999 | $ 2,994

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- Definition This item represents the cost of debt 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 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 Reference 2: 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 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + 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=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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