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 Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Significant non-cash investing and financing activities
Other payable issued for agency acquisitions and purchased customer accounts	$ 3,495	$ 4,350
Estimated acquisition earn-out payables and related charges	25,163	$ 96,845
Contingent payable issued for agency acquisition	24,114
Common stock issued for agency acquisition	4,892
Notes payable assumed for agency acquisition	$ 1,355