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Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | $ 232,142 | $ 261,208
Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 11,230 | 10,915
US Treasury and Government Short-term Debt Securities [Member] |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 22,913 | 55,220
Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 197,999 | 195,073
Recurring basis |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 232,142 | 261,208
Recurring basis | Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 11,230 | 10,915
Recurring basis | US Treasury and Government Short-term Debt Securities [Member] |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 22,913 | 55,220
Recurring basis | Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 197,999 | 195,073
Recurring basis | Quoted Market Prices in Active Markets (Level 1) |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 12,443 | 11,973
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 11,230 | 10,915
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | US Treasury and Government Short-term Debt Securities [Member] |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 0 | 0
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 1,213 | 1,058
Recurring basis | Significant Other Observable Inputs (Level 2) |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 219,699 | 249,235
Recurring basis | Significant Other Observable Inputs (Level 2) | Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 0 | 0
Recurring basis | Significant Other Observable Inputs (Level 2) | US Treasury and Government Short-term Debt Securities [Member] |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 22,913 | 55,220
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 196,786 | 194,015
Recurring basis | Significant Unobservable Inputs (Level 3) |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 0 | 0
Recurring basis | Significant Unobservable Inputs (Level 3) | Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 0 | 0
Recurring basis | Significant Unobservable Inputs (Level 3) | US Treasury and Government Short-term Debt Securities [Member] |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 0 | 0
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | $ 0 | $ 0

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