Business Combinations - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) Acquisition	Dec. 31, 2020 USD ($) Acquisition	Dec. 31, 2019 USD ($) Acquisition	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	19	25
Aggregate purchase price of acquisitions	$ (500)	$ 3,500	$ 4,100
Payments to Acquire Businesses, Gross	424,600	722,500
Goodwill, Acquired During Period	349,973	648,095	328,500
Goodwill currently deductible for income tax purposes	179,100	516,700	245,600
Goodwill related to the recorded earn-out payables	53,000	131,400	82,900
Total revenues related to acquisitions	63,800	93,900	49,100
Income before income taxes related to acquisitions	10,600	7,500	3,400
Maximum Future Contingency payments Acquisitions	484,800
Estimated acquisition earn-out payables	291,047	258,943	161,513	$ 89,924
Other Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	212,600	179,700	143,600
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	78,400	79,200	17,900
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	345,961	299,961	302,600
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	(1,337)	163,070	100
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	5,349	184,956	6,500
Services
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 0	$ 108	$ 19,300
Purchased Customer Accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years	15 years	15 years
Non-compete Agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years	5 years	5 years
Business Combinations - Asset Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Assets and Assumed Certain Liabilities
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	13	20	22
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	2	1	1
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	3	4	4
O'Leary and Remedy
Business Acquisition [Line Items]
Goodwill currently non-deductible for income tax purposes	$ 117,900