Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 203,597	$ 247,459
Contingent consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	18,752	24,301
Deferred consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	184,845	223,158
Level 1
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Level 1 | Contingent consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Level 1 | Deferred consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Level 2
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	184,845	223,158
Level 2 | Contingent consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Level 2 | Deferred consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	184,845	223,158
Level 3
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	18,752	24,301
Level 3 | Contingent consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	18,752	24,301
Level 3 | Deferred consideration
Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 0	$ 0