Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities - Summary of Significant Non-Cash Investing and Financing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Significant non-cash investing and financing activities
Other payables issued for agency acquisitions and purchased customer accounts	$ 12.2	$ 5.6	$ 15.1
Estimated acquisition earn-out payables and related charges	67.2	73.3	75.7
Assumed acquisition earn-out payables	21.1	34.8
Contingent payable issued for agency acquisition			24.1
Common stock issued for agency acquisition	18.3	14.7	9.9
Notes payable assumed for agency acquisition		$ 1.8	$ 1.4
Notes received on the sale of businesses, fixed assets and customer accounts	$ 0.1