Fair Value Measurements - Financial Assets And Liabilities Measured At Fair Value On Recurring Basis (Details) - Fair value, measurements, recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	$ 20,412	$ 36,514
Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	32,518	80,189
Money market funds
Assets:
Fair value of financial assets measured on recurring basis	308,208	137,215
Quoted Prices in Active Markets for Identical Assets (Level 1) | Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Fair value of financial assets measured on recurring basis	308,208	137,215
Significant Other Observable Inputs (Level 2) | Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Fair value of financial assets measured on recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	20,412	36,514
Significant Unobservable Inputs (Level 3) | Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	32,518	80,189
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Fair value of financial assets measured on recurring basis	$ 0	$ 0