Summary of Significant Accounting Policies - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 296,782	$ 368,119
Nicox stock with lockup provisions	29	35
Total assets	296,811	368,154
Purchase consideration payable	201	3,901
Total liabilities	201	3,901
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	296,782	368,119
Nicox stock with lockup provisions	0	0
Total assets	296,782	368,119
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
Nicox stock with lockup provisions	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
Nicox stock with lockup provisions	29	35
Total assets	29	35
Purchase consideration payable	201	3,901
Total liabilities	$ 201	$ 3,901