Summary of Significant Accounting Policies - Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 307,425	$ 200,772
Available-for-sale securities	1,310	1,106
Total assets	308,735	201,878
Purchase consideration payable	4,994	4,994
Total liabilities	4,994	4,994
Quoted Prices in Active Markets for Identical Items (Level 1) | Recurring Measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	307,425	200,772
Available-for-sale securities	1,268	1,074
Total assets	308,693	201,846
Purchase consideration payable	0	0
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Recurring Measurements
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
Significant Unobservable Inputs (Level 3) | Recurring Measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Available-for-sale securities	42	32
Total assets	42	32
Purchase consideration payable	4,994	4,994
Total liabilities	$ 4,994	$ 4,994