Business Combinations - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) Acquisition	Sep. 30, 2019 USD ($) Acquisition	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	16	18
Aggregate purchase price of acquisitions	$ 3,500
Payments to Acquire Businesses, Gross	402,358
Net assets acquired	518,673
Goodwill, Acquired During Period	378,633
Goodwill currently deductible for income tax purposes	281,800
Goodwill related to the recorded earn-out payables	96,800
Total revenues related to acquisitions	52,600
Loss before income taxes related to acquisitions	1,200
Estimated acquisition earn-out payables	252,296	$ 146,603	$ 218,467	$ 161,513	$ 104,976	$ 89,924
Maximum Future Contingency payments Acquisitions	480,200
Asset Purchase Agreement
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	25,000
Expected minimum calculated earn out payments	25,000
Expected maximum deemed earn out payments	$ 25,000
Expected payment period of earn out payments	will be paid in accordance with the asset purchase agreement in the first quarter of calendar year 2022
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 200,400
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	51,900
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	114,250
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	178,332
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	85,943
Services
Business Acquisition [Line Items]
Goodwill, Acquired During Period	108
Goodwill adjusted amount	$ 100
Purchased Customer Accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years
Non-compete Agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years
Business Combinations - Asset Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	13
Business Combinations - Asset Deals and Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	16
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	3
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 10,000