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, 2020	Mar. 31, 2019
Business Combinations [Abstract]
Beginning balance	$ 161,513	$ 89,924
Additions to estimated acquisition earn-out payables	13,978	18,155
Payments for estimated acquisition earn-out payables	(6,814)	(579)
Subtotal	168,677	107,500
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(12,641)	50
Interest expense accretion	1,684	1,160
Net change in earnings from estimated acquisition earn-out payables	(10,957)	1,210
Ending balance	$ 157,720	$ 108,710