Summary of Significant Accounting Policies - Fair Values of the Company's Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 368,119	$ 200,772
Available-for-sale securities	35	1,106
Total assets	368,154	201,878
Purchase consideration payable	3,901	4,994
Total liabilities	3,901	4,994
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	368,119	200,772
Available-for-sale securities	0	1,074
Total assets	368,119	201,846
Purchase consideration payable	0	0
Total liabilities	0	0
Level 2 | Recurring
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
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Available-for-sale securities	35	32
Total assets	35	32
Purchase consideration payable	3,901	4,994
Total liabilities	$ 3,901	$ 4,994