Business Combinations - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) Acquisition	Jun. 30, 2019 USD ($) Acquisition	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	10	14
Aggregate purchase price of acquisitions	$ 3,500
Payments to Acquire Businesses, Gross	291,424
Net assets acquired	378,343
Goodwill, Acquired During Period	277,204
Goodwill currently deductible for income tax purposes	202,400
Goodwill related to the recorded earn-out payables	74,800
Total revenues related to acquisitions	24,300
Income before income taxes related to acquisitions	200
Estimated acquisition earn-out payables	218,467	$ 104,976	$ 157,720	$ 161,513	$ 108,710	$ 89,924
Maximum Future Contingency payments Acquisitions	446,500
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	$ 196,400
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	22,100
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	15,903
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	175,250
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	7
Business Combinations - Asset Deals and Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	10
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