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
	Mar. 31, 2021	Mar. 31, 2020
Business Combinations [Abstract]
Beginning balance	$ 258,943	$ 161,513
Additions to estimated acquisition earn-out payables	14,988	13,978
Payments for estimated acquisition earn-out payables	(15,475)	(6,814)
Subtotal	258,456	168,677
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(2,713)	(12,641)
Interest expense accretion	1,823	1,684
Net change in earnings from estimated acquisition earn-out payables	(890)	(10,957)
Ending balance	$ 257,566	$ 157,720