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Investments | 12 Months Ended
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Dec. 31, 2017
Investments [Abstract] |
Investments | 3. Investments As of December 31, 201 7 and 2016, our investments consist ed of U.S. treasury notes with maturities up to approximately one year and were classified as available-for-sale. Fair value of U.S. treasury notes is measured on a recurring basis based on Level 1 inputs (quoted prices for identical assets in active markets). The following is a summary of available-for-sale securities:
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﻿ | December 31, |  | December 31,
﻿ | 2017 |  | 2016
﻿Amortized cost | $ | 324,875 |  | $ | 455,109
﻿Unrealized gains (losses) |  | (493) |  |  | (218)
﻿Fair value | $ | 324,382 |  | $ | 454,891
﻿ The following is a summary of unrealized gains (losses) on available-for-sale securities recorded in other comprehensive income (loss) in the consolidated statement of comprehensive income: ﻿
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﻿ | Year ended December 31,
﻿ | 2017 |  | 2016 |  | 2015
﻿Unrealized gains (losses) on available-for-sale securities | $ | (274) |  | $ | 2,251 |  | $ | (2,468)
﻿Unrealized gains (losses) on available-for-sale securities, net of tax | $ | (186) |  | $ | 1,402 |  | $ | (1,522)
﻿ Realized gains and losses on available-for-sale securities are recorded in interest and other income on the consolidated statement of income. We had no realized gains or losses for the years ended December 31, 2017 and 2015, and $547 of realized gains on available-for-sale securities for the year ended December 31, 2016. During the year ended December 31, 2015, we recorded an other-than-temporary impairment charge of $244 in interest and other income in the consolidated statement of income in connection with a decline in the fair market value of certain available-for-sale securities. We have elected to fund certain deferred compensation obligations through a rabbi trust, the assets of which are designated as trading securities, as described further in Note 7. “Employee Benefit Plans.”

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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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