Summary of Significant Accounting Policies - Schedule of Fiduciary Assets and Liabilities and Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fiduciary Assets and Liabilities
Cash and cash equivalents	$ 700.3	$ 650.0	$ 693.2
Fiduciary cash	1,602.6	1,383.2
Total assets	14,883.4	13,973.5	9,795.4
Commission, fees and other receivables	789.7	642.9
Fiduciary receivables	1,124.6	881.4
Accounts payable	458.9	286.5
Fiduciary liabilities	2,727.2	2,264.6
Cash flows from operating activities:
Premiums, commissions and fees receivable	(106.4)	(60.9)	(61.9)
Accounts payable	260.0	124.3	54.4
Cash flows from financing activities:
Fiduciary receivables and liabilities, net	$ 188.5	$ 96.2	$ 133.7