Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 318,405	$ 310,764
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	4,063	2,047
Assets, Fair Value Disclosure	562,828	406,195
Fair Value, Measurements, Recurring [Member] | Prepaid expenses and other current assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	3,411	2,956
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	4,989
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	199,302	87,179
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	26,282	2,499
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	2,995
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	3,381	750
Fair Value, Measurements, Recurring [Member] | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	243,786	250,981
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	32,965	42,391
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	146,777	122,689
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,286	5,847
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,756	28,908
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	48,211	45,146
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	5,791	6,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	74,619	59,783
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	57,587	40,812
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	10,033	6,789
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,999	12,005
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		177
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	4,063	2,047
Assets, Fair Value Disclosure	4,063	2,047
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	0	0
Assets, Fair Value Disclosure	257,507	144,330
Fair Value, Measurements, Recurring [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring [Member] | 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	199,302	87,179
Fair Value, Measurements, Recurring [Member] | 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	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	2,995
Fair Value, Measurements, Recurring [Member] | 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	0
Fair Value, Measurements, Recurring [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring [Member] | 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 [Member] | 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	48,211	45,146
Fair Value, Measurements, Recurring [Member] | 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 [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,999	12,005
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	0	0
Assets, Fair Value Disclosure	301,258	259,818
Fair Value, Measurements, Recurring [Member] | 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	3,411	2,956
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	4,989
Fair Value, Measurements, Recurring [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	26,282	2,499
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	3,381	750
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	32,965	42,391
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	146,777	122,689
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,286	5,847
Fair Value, Measurements, Recurring [Member] | 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	6,756	28,908
Fair Value, Measurements, Recurring [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	5,791	6,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	57,587	40,812
Fair Value, Measurements, Recurring [Member] | 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	10,033	6,789
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-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 [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		$ 177