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Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized gains | $ 3 |
Cost |  |
Debt Securities, Available-for-sale [Line Items] |  |
Total Available-for-sale Securities | 60,457 | $ 64,138
Cost | Certificates of deposit | Level 1 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 0 | 250
Marketable Securities with unrecognized losses for more than 12 months | 4,012 | 3,861
Marketable Securities with unrecognized gains | 0 | 0
Total Available-for-sale Securities | 4,012 | 4,111
Cost | U.S. Treasury and agency notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 4,064 | 3,112
Marketable Securities with unrecognized losses for more than 12 months | 19,472 | 23,564
Marketable Securities with unrecognized gains | 2 | 3
Total Available-for-sale Securities | 23,538 | 26,679
Cost | Corporate notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 14,780 | 13,696
Marketable Securities with unrecognized losses for more than 12 months | 9,383 | 12,542
Marketable Securities with unrecognized gains | 1,795 | 0
Total Available-for-sale Securities | 25,958 | 26,238
Cost | Municipal notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 0 | 2,994
Marketable Securities with unrecognized losses for more than 12 months | 6,949 | 4,116
Marketable Securities with unrecognized gains | 0 | 0
Total Available-for-sale Securities | 6,949 | 7,110
Fair Value |  |
Debt Securities, Available-for-sale [Line Items] |  |
Total Available-for-sale Securities | 60,270 | 63,749
Fair Value | Certificates of deposit | Level 1 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 0 | 248
Marketable Securities with unrecognized losses for more than 12 months | 3,991 | 3,812
Marketable Securities with unrecognized gains | 0 | 0
Total Available-for-sale Securities | 3,991 | 4,060
Fair Value | U.S. Treasury and agency notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 4,058 | 3,105
Marketable Securities with unrecognized losses for more than 12 months | 19,391 | 23,415
Marketable Securities with unrecognized gains | 3 | 4
Total Available-for-sale Securities | 23,452 | 26,524
Fair Value | Corporate notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 14,760 | 13,665
Marketable Securities with unrecognized losses for more than 12 months | 9,334 | 12,431
Marketable Securities with unrecognized gains | 1,797 | 0
Total Available-for-sale Securities | 25,891 | 26,096
Fair Value | Municipal notes | Level 2 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Marketable Securities with unrecognized losses for less than 12 months | 0 | 2,982
Marketable Securities with unrecognized losses for more than 12 months | 6,936 | 4,087
Marketable Securities with unrecognized gains | 0 | 0
Total Available-for-sale Securities | $ 6,936 | $ 7,069

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- Definition 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). + References 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 + Details
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