Investments and Fair Value Measurements Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 172,690	$ 311,979
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	565,892	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	3,099	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	390,103	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	164,629	272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	32,656	58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	84,042	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	13,972	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	32,937	60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,022	4,386
Fair Value, Measurements, Recurring | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	8,061	39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,129	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	6,932	14,955
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	423,040	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	390,103	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
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		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	32,937	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	142,852	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	3,099	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		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		2,016
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	32,656	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	84,042	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	13,972	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	1,022	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	1,129	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	6,932	14,955
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	0	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	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		4,476
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	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	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	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	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	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	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	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	$ 0