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Basis of Presentation and Summary of Significant Accounting Policies (Investment Categories) (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Summary of investment categories |  |
Amortized cost | $ 167,057 | $ 120,758
Gross unrealized gains | 2,518 | 1,564
Gross unrealized losses | (55) | (26)
Estimated fair value | 169,520 | 122,296
Bank deposits |  |
Summary of investment categories |  |
Amortized cost | 77,307 | 40,715
Gross unrealized gains | 12 | 19
Gross unrealized losses | (5) | 0
Estimated fair value | 77,314 | 40,734
Corporate bonds |  |
Summary of investment categories |  |
Amortized cost | 50,833 | 11,031
Gross unrealized gains | 3 | 0
Gross unrealized losses | (34) | (5)
Estimated fair value | 50,802 | 11,026
Commercial paper |  |
Summary of investment categories |  |
Amortized cost | 23,176 | 33,074
Gross unrealized gains | 3 | 2
Gross unrealized losses | (2) | (9)
Estimated fair value | 23,177 | 33,067
U.S. Government bonds |  |
Summary of investment categories |  |
Amortized cost | 8,490 | 7,508
Gross unrealized gains | 2 | 0
Gross unrealized losses | (6) | (1)
Estimated fair value | 8,486 | 7,507
Agency bonds |  |
Summary of investment categories |  |
Amortized cost | 4,977 | 7,294
Gross unrealized gains | 0 | 1
Gross unrealized losses | 0 | 0
Estimated fair value | 4,977 | 7,295
Municipal Bonds |  |
Summary of investment categories |  |
Amortized cost | 2,020 | 19,624
Gross unrealized gains | 0 | 0
Gross unrealized losses | (8) | (11)
Estimated fair value | 2,012 | 19,613
Corporate equity securities |  |
Summary of investment categories |  |
Amortized cost | 254 | 1,512
Gross unrealized gains | 2,498 | 1,542
Gross unrealized losses | 0 | 0
Estimated fair value | $ 2,752 | $ 3,054

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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=75030427&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=82897076&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=82896770&loc=d3e26610-111562 + Details
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- 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 + Details
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