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Marketable Securities | 3 Months Ended
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Mar. 31, 2017
Marketable Securities [Abstract] |
Marketable Securities | ﻿ Note 4. marketable securities ﻿ The amortized cost and fair value of marketable securities were as follows (in thousands): ﻿
﻿ |  |  |  |  |  |  |  |  |  |  |  |  |
﻿As of March 31, 2017 |  |  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
﻿ |  |  |  |  |  |  |  |  |  |  |  |  |
﻿Corporate bonds |  | $ | 120,898 |  | $ | 76 |  | $ | (81) |  | $ | 120,893 |
﻿Certificates of deposit |  |  | 43,198 |  |  | - |  |  | - |  |  | 43,198 |
﻿Asset backed securities |  |  | 37,610 |  |  | 6 |  |  | (16) |  |  | 37,600 |
﻿Commercial paper |  |  | 17,013 |  |  | - |  |  | - |  |  | 17,013 |
﻿U.S. government bonds |  |  | 16,644 |  |  | 1 |  |  | (15) |  |  | 16,630 |
﻿Agency bonds |  |  | 4,600 |  |  | - |  |  | (1) |  |  | 4,599 |
﻿Total marketable securities |  | $ | 239,963 |  | $ | 83 |  | $ | (113) |  | $ | 239,933 |
﻿ |  |  |  |  |  |  |  |  |  |  |  |  |
﻿As of December 31, 2016 |  |  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
﻿ |  |  |  |  |  |  |  |  |  |  |  |  |
﻿Corporate bonds |  | $ | 130,833 |  | $ | 40 |  | $ | (102) |  | $ | 130,771 |
﻿Certificates of deposit |  |  | 40,400 |  |  | - |  |  | - |  |  | 40,400 |
﻿Asset backed securities |  |  | 27,290 |  |  | 25 |  |  | - |  |  | 27,315 |
﻿Commercial paper |  |  | 20,228 |  |  | - |  |  | - |  |  | 20,228 |
﻿U.S. government bonds |  |  | 12,244 |  |  | 1 |  |  | (14) |  |  | 12,231 |
﻿Agency bonds |  |  | 4,600 |  |  | 4 |  |  | - |  |  | 4,604 |
﻿Municipal bonds |  |  | 1,400 |  |  | - |  |  | - |  |  | 1,400 |
﻿Total marketable securities |  | $ | 236,995 |  | $ | 70 |  | $ | (116) |  | $ | 236,949 |
﻿ As of March 31, 2017, unrealized losses on marketable securities that have been in a continuous loss position for more than twelve months were not material. Our portfolio of marketable securities had an average AA- credit rating as of March 31, 2017. There were no marketable securities that we consider to be other-than-temporarily impaired as of March 31, 2017. ﻿ Remaining effective maturities of marketable securities were as follows (in thousands): ﻿ ﻿
﻿ |  |  |  |  |  |  |
﻿As of March 31, 2017 |  |  | Amortized Cost |  |  | Fair Value |
﻿ |  |  |  |  |  |  |
﻿Due in one year or less |  | $ | 144,581 |  | $ | 144,547 |
﻿Due after one year through three years |  |  | 95,382 |  |  | 95,386 |
﻿ |  | $ | 239,963 |  | $ | 239,933 |
﻿ Our investment strategy is to buy short-duration marketable securities with a high credit rating. Some of our marketable securities have call features that can effectively shorten the lifespan from the contractual maturity date. We use the effective maturity date to measure the duration of the marketable securities.

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