Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Combinations [Abstract]
Beginning balance	$ 161,513	$ 89,924	$ 36,175
Additions to estimated acquisition earn-out payables from new acquisitions	131,397	82,872	77,377
Payments for estimated acquisition earn-out payables	(29,509)	(9,917)	(26,597)
Subtotal	263,401	162,879	86,955
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(11,814)	(7,298)	603
Interest expense accretion	7,356	5,932	2,366
Net change in earnings from estimated acquisition earn- out payables	(4,458)	(1,366)	2,969
Ending balance	$ 258,943	$ 161,513	$ 89,924