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Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | $ 261,208 | $ 121,151
Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 10,915 | 10,681
US Treasury and Government Short-term Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 55,220 | 48,292
Corporate notes, bonds and commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 195,073 | 62,178
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 261,208 | 121,151
Fair Value, Measurements, Recurring [Member] | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 10,915 | 10,681
Fair Value, Measurements, Recurring [Member] | US Treasury and Government Short-term Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 55,220 | 48,292
Fair Value, Measurements, Recurring [Member] | Corporate notes, bonds and commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 195,073 | 62,178
Fair Value, Measurements, Recurring [Member] | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 11,973 | 10,984
Fair Value, Measurements, Recurring [Member] | Level 1 | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 10,915 | 10,681
Fair Value, Measurements, Recurring [Member] | Level 1 | US Treasury and Government Short-term Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 1 | Corporate notes, bonds and commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 1,058 | 303
Fair Value, Measurements, Recurring [Member] | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 249,235 | 110,167
Fair Value, Measurements, Recurring [Member] | Level 2 | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 2 | US Treasury and Government Short-term Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 55,220 | 48,292
Fair Value, Measurements, Recurring [Member] | Level 2 | Corporate notes, bonds and commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 194,015 | 61,875
Fair Value, Measurements, Recurring [Member] | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 | US Treasury and Government Short-term Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 | Corporate notes, bonds and commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total available-for-sale securities | $ 0 | $ 0

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