Business Combinations - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) Acquisition	Sep. 30, 2018 USD ($) Acquisition	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	18	18
Aggregate purchase price of acquisitions	$ (6,600)
Payments to Acquire Businesses, Gross	291,610
Net assets acquired	392,536
Goodwill, Acquired During Period	275,307
Goodwill currently deductible for income tax purposes	203,800
Goodwill related to the recorded earn-out payables	71,500
Total revenues related to acquisitions	31,000
Income before income taxes related to acquisitions	300
Estimated acquisition earn-out payables	146,603	$ 65,308	$ 104,976	$ 89,924	$ 52,540	$ 36,175
Maximum Future Contingency payments Acquisitions	310,600
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	122,900
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	23,700
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	249,691
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	74
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	6,189
Services
Business Acquisition [Line Items]
Goodwill, Acquired During Period	19,353
Goodwill adjusted amount	$ 19,300
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	17
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 10,000