Fair Value Measurements - Financial Assets And Liabilities Measured At Fair Value On Recurring Basis (Details) - Fair value, measurements, recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fair value of financial assets measured on recurring basis	$ 641,981	$ 137,953
Liabilities:
Fair value of financial liabilities measured on recurring basis	67,842	116,703
Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	28,305	36,514
Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	39,537	80,189
Money market funds
Assets:
Fair value of financial assets measured on recurring basis	641,012	137,215
Equity securities
Assets:
Fair value of financial assets measured on recurring basis	969	738
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Fair value of financial assets measured on recurring basis	641,981	137,953
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—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	641,012	137,215
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets:
Fair value of financial assets measured on recurring basis	969	738
Significant Other Observable Inputs (Level 2)
Assets:
Fair value of financial assets measured on recurring basis	0	0
Liabilities:
Fair value of financial liabilities measured on recurring basis	0	0
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 Other Observable Inputs (Level 2) | Equity securities
Assets:
Fair value of financial assets measured on recurring basis	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Fair value of financial assets measured on recurring basis	0	0
Liabilities:
Fair value of financial liabilities measured on recurring basis	67,842	116,703
Significant Unobservable Inputs (Level 3) | Acquisition-related contingent consideration—current
Liabilities:
Fair value of financial liabilities measured on recurring basis	28,305	36,514
Significant Unobservable Inputs (Level 3) | Acquisition-related contingent consideration—noncurrent
Liabilities:
Fair value of financial liabilities measured on recurring basis	39,537	80,189
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Fair value of financial assets measured on recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Assets:
Fair value of financial assets measured on recurring basis	$ 0	$ 0