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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Business Combinations [Abstract]
Beginning balance	$ 157,720	$ 108,710	$ 161,513	$ 89,924
Additions to estimated acquisition earn-out payables	60,776	5,811	74,754	23,966
Payments for estimated acquisition earn-out payables	(664)	(6,685)	(7,478)	(7,264)
Subtotal	217,832	107,836	228,789	106,626
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(1,308)	(4,350)	(13,949)	(4,301)
Interest expense accretion	1,943	1,490	3,627	2,651
Net change in earnings from estimated acquisition earn-out payables	635	(2,860)	(10,322)	(1,650)
Ending balance	$ 218,467	$ 104,976	$ 218,467	$ 104,976