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, 2022	Dec. 31, 2021	Dec. 31, 2020
Fiduciary Assets and Liabilities
Cash and cash equivalents	$ 650.0	$ 693.2	$ 656.2
Fiduciary cash	1,383.2	777.0
Total assets	13,973.5	9,795.4	8,966.5
Commission, fees and other receivables	642.9	522.6
Fiduciary receivables	881.4	693.7
Accounts payable	286.5	242.7
Fiduciary liabilities	2,264.6	1,470.7
Cash flows from operating activities:
Premiums, commissions and fees receivable	(60.9)	(61.9)	(2.0)
Accounts payable	124.3	54.4	111.6
Cash flows from financing activities:
Fiduciary receivables and liabilities, net	$ 96.2	133.7	8.7
Total restated changes in cash flows		126.2	118.3
As Reported
Cash flows from operating activities:
Premiums, commissions and fees receivable		(72.8)	(135.4)
Premiums payable to insurance companies		128.0	158.8
Premium deposits and credits due customers		19.6	(12.9)
Accounts payable		51.4	107.8
Cash flows from financing activities:
Total restated changes in cash flows		126.2	118.3
Change in Presentation
Cash flows from operating activities:
Premiums, commissions and fees receivable		10.9	133.4
Premiums payable to insurance companies		(128.0)	(158.8)
Premium deposits and credits due customers		(19.6)	12.9
Accounts payable		3.0	3.8
Cash flows from financing activities:
Fiduciary receivables and liabilities, net		133.7	$ 8.7
Fiduciary Cash
Fiduciary Assets and Liabilities
Cash and cash equivalents		693.2
Fiduciary cash		777.0
Total assets		1,470.2
Fiduciary Cash | As Reported
Fiduciary Assets and Liabilities
Cash and cash equivalents		887.0
Restricted cash and investments		583.2
Total assets		1,470.2
Fiduciary Cash | Change in Presentation
Fiduciary Assets and Liabilities
Cash and cash equivalents		(193.8)
Restricted cash and investments		(583.2)
Fiduciary cash		777.0
Fiduciary Receivable
Fiduciary Assets and Liabilities
Total assets		1,216.3
Commission, fees and other receivables		522.6
Fiduciary receivables		693.7
Fiduciary Receivable | As Reported
Fiduciary Assets and Liabilities
Total assets		1,216.3
Premiums, commissions and fees receivable		1,216.3
Fiduciary Receivable | Change in Presentation
Fiduciary Assets and Liabilities
Premiums, commissions and fees receivable		(1,216.3)
Commission, fees and other receivables		522.6
Fiduciary receivables		693.7
Fiduciary Liability
Fiduciary Assets and Liabilities
Accounts payable		242.7
Fiduciary liabilities		1,470.7
Total		1,713.4
Fiduciary Liability | As Reported
Fiduciary Assets and Liabilities
Premiums payable to insurance companies		1,384.6
Premium deposits and credits due customers		122.4
Accounts payable		206.4
Total		1,713.4
Fiduciary Liability | Change in Presentation
Fiduciary Assets and Liabilities
Premiums payable to insurance companies		(1,384.6)
Premium deposits and credits due customers		(122.4)
Accounts payable		36.3
Fiduciary liabilities		$ 1,470.7