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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Combinations [Abstract]
Beginning balance	$ 104,976	$ 52,540	$ 89,924	$ 36,175
Additions to estimated acquisition earn-out payables	47,498	29,646	71,464	51,717
Payments for estimated acquisition earn-out payables	(601)	(16,521)	(7,865)	(25,112)
Subtotal	151,873	65,665	153,523	62,780
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(6,573)	(928)	(10,873)	945
Interest expense accretion	1,303	571	3,953	1,583
Net change in earnings from estimated acquisition earn-out payables	(5,270)	(357)	(6,920)	2,528
Ending balance	$ 146,603	$ 65,308	$ 146,603	$ 65,308