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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Business Combinations [Abstract]
Beginning balance	$ 257,566	$ 157,720	$ 258,943	$ 161,513
Additions to estimated acquisition earn-out payables	10,097	60,776	25,085	74,754
Payments for estimated acquisition earn-out payables	(24,475)	(664)	(39,949)	(7,478)
Subtotal	243,188	217,832	244,079	228,789
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(3,326)	(1,308)	(6,040)	(13,949)
Interest expense accretion	1,722	1,943	3,545	3,627
Net change in earnings from estimated acquisition earn-out payables	(1,604)	635	(2,495)	(10,322)
Foreign currency translation adjustments during the year	432		432
Ending balance	$ 242,016	$ 218,467	$ 242,016	$ 218,467