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Fair Value of Financial Instruments (Details) - USD ($) | Mar. 31, 2019 | Dec. 31, 2018
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Financial assets subject to fair value measurements and the necessary disclosures |  |
Equity Method Investment, Ownership Percentage | 27.70% |
Total available-for-sale securities | $ 264,477,000 | $ 226,708,000
Equity Method Investments | 3,200,000.0 |
Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 10,849,000 | 10,080,000
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 | 41,479,000 | 32,630,000
Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 212,149,000 | 183,998,000
Recurring basis |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 264,477,000 | 226,708,000
Recurring basis | Money market funds |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 10,849,000 | 10,080,000
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 | 41,479,000 | 32,630,000
Recurring basis | Corporate notes, bonds and commercial paper |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 212,149,000 | 183,998,000
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 | 10,849,000 | 10,080,000
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 | 10,849,000 | 10,080,000
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 | 0 | 0
Recurring basis | Significant Other Observable Inputs (Level 2) |  |
Financial assets subject to fair value measurements and the necessary disclosures |  |
Total available-for-sale securities | 253,628,000 | 216,628,000
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 | 41,479,000 | 32,630,000
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 | 212,149,000 | 183,998,000
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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