Summary of Significant Accounting Policies - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 345,380	$ 200,772
Available-for-sale securities	33	1,106
Total assets	345,413	201,878
Purchase consideration payable	4,994	4,994
Total liabilities	4,994	4,994
Recurring Measurements | Quoted Prices in Active Markets for Identical Items (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	345,380	200,772
Available-for-sale securities	0	1,074
Total assets	345,380	201,846
Purchase consideration payable	0	0
Total liabilities	0	0
Recurring Measurements | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Available-for-sale securities	0	0
Total assets	0	0
Purchase consideration payable	0	0
Total liabilities	0	0
Recurring Measurements | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Available-for-sale securities	33	32
Total assets	33	32
Purchase consideration payable	4,994	4,994
Total liabilities	$ 4,994	$ 4,994