Business Combinations - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) Acquisition	Dec. 31, 2019 USD ($) Acquisition	Dec. 31, 2018 USD ($) Acquisition	Dec. 12, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	25	27
Aggregate purchase price of acquisitions	$ (3,500,000)	$ 4,100,000	$ 21,400
Payments to Acquire Businesses, Gross	722,515,000	356,260,000	934,928,000
Goodwill, Acquired During Period	648,095,000	328,546,000	717,700,000
Goodwill currently deductible for income tax purposes	516,700,000	245,600,000	640,300,000
Goodwill related to the recorded earn-out payables	131,400,000	82,900,000	77,400,000
Total revenues related to acquisitions	93,900,000	49,100,000	82,400,000
Income before income taxes related to acquisitions	7,500,000	3,400,000	6,300,000
Maximum Future Contingency payments Acquisitions	544,700,000
Estimated acquisition earn-out payables	258,943,000	161,513,000	89,924,000	$ 89,900,000	$ 36,175,000
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	179,700,000	143,600,000		68,800,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	79,200,000	17,900,000		$ 21,100,000
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	299,961,000	302,640,000	676,900,000
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	163,070,000	74,000	18,700,000
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	184,956,000	6,479,000	5,500,000
Services
Business Acquisition [Line Items]
Goodwill, Acquired During Period	108,000	19,353,000
Goodwill adjusted amount	$ 100,000	$ 19,300,000	$ 16,500,000
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	20	22	20
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1	1	3
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	4	4	1