Acquisitions - Narrative (Details) $ in Thousands	1 Months Ended		3 Months Ended					9 Months Ended		12 Months Ended
	Oct. 31, 2021 USD ($)	Oct. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) business	Jun. 30, 2021 USD ($) business	Mar. 31, 2021 USD ($) business	Dec. 31, 2020 USD ($) business	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Acquisitions
Cash paid, net of cash acquired								$ 39,253	$ 16,854
Acquisition-related costs			$ 200					700
General and Administrative Expense
Acquisitions
Acquisition-related costs			$ 600					1,400
2021 Acquisitions
Acquisitions
Number of businesses acquired | business			3	2	3
Number of business' assets acquired | business			1		1
Aggregate purchase price			$ 6,200	$ 21,000	$ 24,200			51,438
Cash paid, net of cash acquired			3,900	16,000	19,200			39,104
Issuance of Seller Notes at fair value			1,500	4,900	4,000			10,442
Additional consideration issued			800	$ 100	$ 1,000			1,892
Operating right-of-use assets			4,000					4,000
Operating lease liabilities			$ 4,000					$ 4,000
2020 Acquisitions
Acquisitions
Number of businesses acquired | business						4
Aggregate purchase price						$ 7,100	$ 46,200			$ 53,262
Cash paid, net of cash acquired						4,900	16,800			21,709
Issuance of Seller Notes at fair value						1,900	21,900			23,766
Additional consideration issued		$ 3,600				300	4,000			4,319
Operating right-of-use assets						5,500				5,500
Operating lease liabilities						$ 5,500				5,500
Deferred payment obligation at fair value							$ 3,500			$ 3,468
Payment period of liabilities incurred							3 years
Payment period following acquisition							3 years
Consideration through settlement of amounts due from acquiree		$ 400
2020 Acquisitions | Subsequent Event
Acquisitions
Repayments of debt	$ 18,100
Remaining liabilities payable	$ 3,800