Summary of Significant Accounting Policies - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 275,346	$ 368,119
Nicox stock with lockup provisions	23	35
Total assets	275,369	368,154
Purchase consideration payable		3,901
Total liabilities		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	275,346	368,119
Nicox stock with lockup provisions	0	0
Total assets	275,346	368,119
Purchase consideration payable		0
Total liabilities		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
Total liabilities		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	23	35
Total assets	$ 23	35
Purchase consideration payable		3,901
Total liabilities		$ 3,901