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Marketable Securities | 3 Months Ended
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Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | Marketable Securities At March 31, 2018, marketable securities consisted of the following (in thousands):
| Amortized Cost |  | Unrealized Gains |  | Unrealized Losses |  | Aggregate Fair Value
U.S. treasury debt securities | $ | 3,088 |  | $ | — |  | $ | (8) |  | $ | 3,080
U.S. corporate debt securities | 12,827 |  | — |  | (106) |  | 12,721
Money market funds | 54,830 |  | — |  | — |  | 54,830
| $ | 70,745 |  | $ | — |  | $ | (114) |  | $ | 70,631
Included in cash and cash equivalents | $ | 54,830 |  | $ | — |  | $ | — |  | $ | 54,830
Included in marketable securities | $ | 15,915 |  | $ | — |  | $ | (114) |  | $ | 15,801
At December 31, 2017, marketable securities consisted of the following (in thousands):
| Amortized Cost |  | Unrealized Gains |  | Unrealized Losses |  | Aggregate Fair Value
U.S. treasury debt securities | $ | 3,083 |  | $ | — |  | $ | (8) |  | $ | 3,075
U.S. corporate debt securities | 13,350 |  | — |  | (61) |  | 13,289
Money market funds | 49,452 |  | — |  | — |  | 49,452
| $ | 65,885 |  | $ | — |  | $ | (69) |  | $ | 65,816
Included in cash and cash equivalents | $ | 49,452 |  | $ | — |  | $ | — |  | $ | 49,452
Included in marketable securities | $ | 16,433 |  | $ | — |  | $ | (69) |  | $ | 16,364
The following table presents gross unrealized losses and fair values for those marketable securities that were in an unrealized loss position as of March 31, 2018, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (in thousands):
| As of March 31, 2018
| Less than 12 months |  | 12 months or greater
| Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss
U.S. treasury debt securities | $ | 1,980 |  | $ | (7) |  | $ | 1,100 |  | $ | (1)
U.S. corporate debt securities | 12,222 |  | (106) |  | — |  | —
Total | $ | 14,202 |  | $ | (113) |  | $ | 1,100 |  | $ | (1)
We do not believe any of the unrealized losses represented an other-than-temporary impairment based on our evaluation of available evidence, which includes our intent as of March 31, 2018 to hold these investments until the cost basis is recovered.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=4738149 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2289146 + Details
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