Fair Value Measurements - Liability Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - Contingent Consideration - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 69,100	$ 93,204	$ 76,709	$ 82,236
Contingent consideration liability recorded as the result of acquisitions		1,203		9,583
Contingent consideration expense (benefit)	$ (4,356)	$ 273	$ 884	$ 3,473
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	us-gaap:BusinessCombinationContingentConsiderationArrangementsContingentConsiderationBenefit	us-gaap:BusinessCombinationContingentConsiderationArrangementsContingentConsiderationBenefit	us-gaap:BusinessCombinationContingentConsiderationArrangementsContingentConsiderationBenefit	us-gaap:BusinessCombinationContingentConsiderationArrangementsContingentConsiderationBenefit
Contingent payments made	$ (130)	$ (15,072)	$ (12,991)	$ (15,684)
Effect of foreign exchange	51		63
Ending balance	$ 64,665	$ 79,608	$ 64,665	$ 79,608