Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 174,952	$ 311,979
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	438,298	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	3,114	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	4,859	4,476
Fair Value, Measurements, Recurring | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	255,373	253,155
Fair Value, Measurements, Recurring | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		7,246
Fair Value, Measurements, Recurring | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		2,016
Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	164,740	272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	20,446	58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	106,251	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	8,947	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	28,095	60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,001	4,386
Fair Value, Measurements, Recurring | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	10,212	39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,246	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	6,966	14,955
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	283,468	313,735
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term notes receivable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term notes receivable	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	255,373	253,155
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	28,095	60,580
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (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 | Quoted Prices in Active Markets for Identical Assets (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 | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	149,971	263,736
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Prepaid expenses and other current assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	3,114	3,075
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Short-term notes receivable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term notes receivable	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (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 | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		7,246
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		2,016
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	20,446	58,502
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	106,251	145,557
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	8,947	3,006
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (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 | Significant Other Observable Inputs (Level 2) | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,001	4,386
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,246	24,993
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,966	14,955
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	4,859	4,476
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | 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 | Significant Other Observable Inputs (Level 3) | Short-term notes receivable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term notes receivable	4,859	4,476
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | 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 | Significant Other Observable Inputs (Level 3) | 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 | Significant Other Observable Inputs (Level 3) | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 3) | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 0	$ 0