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Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Cash and Cash Equivalents [Line Items] |  |  |  |
Cash and cash equivalents | $ 60,333 | $ 51,281 | $ 58,750 | $ 101,131
Available-for-sale Securities [Abstract] |  |  |  |
Unrealized Gains | 0 | 1 |  |
Unrealized Losses | (69) | (12) |  |
Cash and cash equivalents and available-for-sale securities, amortized cost | 65,885 | 54,942 |  |
Cash and cash equivalents and available-for-sale securities | 65,816 | 54,931 |  |
U.S. treasury debt securities |  |  |  |
Available-for-sale Securities [Abstract] |  |  |  |
Amortized Cost | 3,083 | 3,503 |  |
Unrealized Gains | 0 | 0 |  |
Unrealized Losses | (8) | (5) |  |
Aggregate Fair Value | 3,075 | 3,498 |  |
U.S. corporate debt securities |  |  |  |
Available-for-sale Securities [Abstract] |  |  |  |
Amortized Cost | 13,350 | 7,943 |  |
Unrealized Gains | 0 | 1 |  |
Unrealized Losses | (61) | (7) |  |
Aggregate Fair Value | 13,289 | 7,937 |  |
Money market funds |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Cash and cash equivalents | 49,452 | 43,496 |  |
Cash and cash equivalents, aggregate fair value | 49,452 | 43,496 |  |
Cash and Cash Equivalents | Money market funds |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Cash and cash equivalents | 49,452 | 43,496 |  |
Cash and cash equivalents, aggregate fair value | 49,452 | 43,496 |  |
Marketable Securities | U.S. corporate debt securities |  |  |  |
Available-for-sale Securities [Abstract] |  |  |  |
Amortized Cost | 16,433 | 11,446 |  |
Unrealized Gains | 0 | 1 |  |
Unrealized Losses | (69) | (12) |  |
Aggregate Fair Value | $ 16,364 | $ 11,435 |  |

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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=82897076&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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