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Fair Value Measurements - Schedule of Assets Measured on a Recurring and Nonrecurring Basis (Details) - Recurring fair value measurements: - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 95,894 $ 241,936
Short-term investments 163,546 167,479
Long-term investments 156,060 125,549
Money market mutual funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 85,905 236,936
U.S. government and agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 9,989 5,000
Short-term investments 120,653 112,076
Long-term investments 100,817 92,464
Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 40,940 47,681
Long-term investments 47,510 27,812
Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 1,953 7,722
Long-term investments 7,733 5,273
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 95,894 241,936
Short-term investments 84,591 90,703
Long-term investments 68,950 49,651
Level 1 | Money market mutual funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 85,905 236,936
Level 1 | U.S. government and agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 9,989 5,000
Short-term investments 84,591 90,703
Long-term investments 68,950 49,651
Level 1 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Long-term investments 0 0
Level 1 | Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Long-term investments 0 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term investments 78,955 76,776
Long-term investments 87,110 75,898
Level 2 | Money market mutual funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Level 2 | U.S. government and agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term investments 36,062 21,373
Long-term investments 31,867 42,813
Level 2 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 40,940 47,681
Long-term investments 47,510 27,812
Level 2 | Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 1,953 7,722
Long-term investments 7,733 5,273
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term investments 0 0
Long-term investments 0 0
Level 3 | Money market mutual funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Level 3 | U.S. government and agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term investments 0 0
Long-term investments 0 0
Level 3 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Long-term investments 0 0
Level 3 | Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Long-term investments $ 0 $ 0