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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Combinations [Abstract]
Beginning balance	$ 218,467	$ 104,976	$ 161,513	$ 89,924
Additions to estimated acquisition earn-out payables	22,091	47,498	96,845	71,464
Payments for estimated acquisition earn-out payables	(3,580)	(601)	(11,058)	(7,865)
Subtotal	236,978	151,873	247,300	153,523
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	13,433	(6,573)	(516)	(10,873)
Interest expense accretion	1,885	1,303	5,512	3,953
Net change in earnings from estimated acquisition earn-out payables	15,318	(5,270)	4,996	(6,920)
Ending balance	$ 252,296	$ 146,603	$ 252,296	$ 146,603