Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Combinations [Abstract]
Beginning balance	$ 242,016	$ 218,467	$ 258,943	$ 161,513
Additions to estimated acquisition earn-out payables	79	22,091	25,163	96,845
Payments for estimated acquisition earn-out payables	(1,913)	(3,580)	(41,862)	(11,058)
Subtotal	240,182	236,978	242,244	247,300
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	21,855	13,433	15,814	(516)
Interest expense accretion	1,283	1,885	4,829	5,512
Net change in earnings from estimated acquisition earn-out payables	23,138	15,318	20,643	4,996
Foreign currency translation adjustments during the year	(406)		27
Ending balance	$ 262,914	$ 252,296	$ 262,914	$ 252,296