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Investments and Fair Value Measurements Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Apr. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments $ 192,388   $ 311,979
Fair Value, Measurements, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Assets, Fair Value Disclosure 461,117   581,947
Fair Value, Measurements, Recurring | Prepaid expenses and other current assets      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Other assets 3,143   3,075
Fair Value, Measurements, Recurring | Short-term notes receivable      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term notes receivable     4,476
Fair Value, Measurements, Recurring | Cash Equivalents | Money market funds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 256,689   253,155
Fair Value, Measurements, Recurring | Cash Equivalents | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 6,883   7,246
Fair Value, Measurements, Recurring | Cash Equivalents | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 2,014   2,016
Fair Value, Measurements, Recurring | Short-term Investments      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 184,297   272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 24,675   58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 75,368   145,557
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 11,634   3,006
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 72,600   60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 20   4,386
Fair Value, Measurements, Recurring | Long-term Investments      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 8,091   39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 1,130   24,993
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 6,961   14,955
Fair Value, Measurements, Recurring | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Assets, Fair Value Disclosure 329,289   313,735
Fair Value, Measurements, Recurring | Level 1 | Prepaid expenses and other current assets      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Other assets 0   0
Fair Value, Measurements, Recurring | Level 1 | Short-term notes receivable      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term notes receivable     0
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Money market funds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 256,689   253,155
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 0   0
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 0   0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | U.S. government treasury bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 72,600   60,580
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Certificates of deposit      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Assets, Fair Value Disclosure 131,828   263,736
Fair Value, Measurements, Recurring | Level 2 | Prepaid expenses and other current assets      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Other assets 3,143   3,075
Fair Value, Measurements, Recurring | Level 2 | Short-term notes receivable      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term notes receivable     0
Fair Value, Measurements, Recurring | Level 2 | Cash Equivalents | Money market funds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 0   0
Fair Value, Measurements, Recurring | Level 2 | Cash Equivalents | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 6,883   7,246
Fair Value, Measurements, Recurring | Level 2 | Cash Equivalents | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents 2,014   2,016
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 24,675   58,502
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 75,368   145,557
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 11,634   3,006
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | U.S. government treasury bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 0   0
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Certificates of deposit      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 20   4,386
Fair Value, Measurements, Recurring | Level 2 | Long-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments 1,130   24,993
Fair Value, Measurements, Recurring | Level 2 | Long-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments $ 6,961   14,955
Fair Value, Measurements, Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Assets, Fair Value Disclosure     4,476
Fair Value, Measurements, Recurring | Level 3 | Prepaid expenses and other current assets      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Other assets     0
Fair Value, Measurements, Recurring | Level 3 | Short-term notes receivable      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term notes receivable   $ 4,900 4,476
Fair Value, Measurements, Recurring | Level 3 | Cash Equivalents | Money market funds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents     0
Fair Value, Measurements, Recurring | Level 3 | Cash Equivalents | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents     0
Fair Value, Measurements, Recurring | Level 3 | Cash Equivalents | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Cash equivalents     0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | U.S. government treasury bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | Certificates of deposit      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Long-term Investments | Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     0
Fair Value, Measurements, Recurring | Level 3 | Long-term Investments | U.S. government agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Investments     $ 0