Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities Carried at Fair Value:
Acquired contingent consideration	$ 112,722	$ 72,100
Level 1 | Recurring basis
Assets Carried at Fair Value:
Cash equivalents	9,163	18,514
Level 3 | Recurring basis
Liabilities Carried at Fair Value:
Acquired contingent consideration	$ 113,000
Acquired contingent consideration		$ 72,100