Summary of Significant Accounting Policies - Fair Values of the Company's Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 224,868	$ 368,119
Nicox stock with lockup provisions	18	35
Total assets	224,886	368,154
Purchase consideration payable		3,901
Total liabilities		3,901
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	224,868	368,119
Nicox stock with lockup provisions	0	0
Total assets	224,868	368,119
Purchase consideration payable		0
Total liabilities		0
Level 2 | Recurring
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
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Nicox stock with lockup provisions	18	35
Total assets	$ 18	35
Purchase consideration payable		3,901
Total liabilities		$ 3,901