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Marketable Securities | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | Marketable Securities Available-for-sale marketable securities as of December 31, 2017 and 2016 were as follows (in thousands):
| December 31, 2017
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. Treasury securities | $ | 3,995 |  |  | $ | — |  |  | $ | (6 | ) |  | $ | 3,989 |
Government-sponsored enterprises | 11,998 |  |  | — |  |  | (7 | ) |  | 11,991 |
Corporate debt securities | 77,462 |  |  | 2 |  |  | (50 | ) |  | 77,414 |
Total | $ | 93,455 |  |  | $ | 2 |  |  | $ | (63 | ) |  | $ | 93,394 |
| December 31, 2016
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. Treasury securities | $ | 4,826 |  |  | $ | — |  |  | $ | (1 | ) |  | $ | 4,825 |
Government-sponsored enterprises | 29,764 |  |  | 5 |  |  | (10 | ) |  | 29,759 |
Corporate debt securities | 166,376 |  |  | 51 |  |  | (127 | ) |  | 166,300 |
Total | $ | 200,966 |  |  | $ | 56 |  |  | $ | (138 | ) |  | $ | 200,884 |
All of the Company's available-for-sale securities held at December 31, 2017 had maturity dates of less than one year. The contractual maturities of the available-for-sale marketable securities as of December 31, 2016 were as follows (in thousands):
| Amortized Cost |  | Fair Value
Mature in one year or less | $ | 192,985 |  |  | $ | 192,898 |
Mature between one and five years | 7,981 |  |  | 7,986 |
Total | $ | 200,966 |  |  | $ | 200,884 |
All available-for-sale securities in an unrealized loss position as of December 31, 2017 and 2016 were in a loss position for less than twelve months. There were no unrealized losses at December 31, 2017 or 2016 that the Company determined to be other-than-temporary. The Company recorded interest income of $2.4 million, $2.3 million and $0.2 million during the years ended December 31, 2017, 2016 and 2015, respectively, which is included in other income.

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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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