Basis of Financial Reporting - Summary of Fiduciary Assets and Liabilities and Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Fiduciary Assets and Liabilities
Cash and cash equivalents	$ 1,694.5	$ 511.7	$ 693.2
Fiduciary cash	731.6		777.0
Total assets	11,272.9	8,910.8	9,795.4
Commission, fees and other receivables	613.5		522.6
Fiduciary cash	706.1		693.7
Accounts payable	230.8		242.7
Fiduciary liabilities	1,437.7		1,470.7
Cash flows from operating activities:
Premiums, commissions and fees receivable	(87.8)	(88.6)
Accounts payable	22.2	70.4
Cash flows from financing activities:
Fiduciary receivables and liabilities, net	$ (85.2)	(12.9)
Total restated changes in cash flows		(31.1)
As Reported
Cash flows from operating activities:
Premiums, commissions and fees receivable		(22.0)
Premiums payable to insurance companies		(103.9)
Premium deposits and credits due customers		25.9
Accounts payable		68.9
Cash flows from financing activities:
Total restated changes in cash flows		(31.1)
Change in Presentation
Cash flows from operating activities:
Premiums, commissions and fees receivable		(66.6)
Premiums payable to insurance companies		103.9
Premium deposits and credits due customers		(25.9)
Accounts payable		1.5
Cash flows from financing activities:
Fiduciary receivables and liabilities, net		$ (12.9)
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 cash			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 cash			693.7
Fiduciary Liability
Fiduciary Assets and Liabilities
Accounts payable			242.7
Fiduciary liabilities			1,470.7
Liabilities, Total			1,713.4
Fiduciary Liability | As Reported
Fiduciary Assets and Liabilities
Premium payable to insurance companies			1,384.6
Premium deposits and credits due customers			122.4
Accounts payable			206.4
Liabilities, Total			1,713.4
Fiduciary Liability | Change in Presentation
Fiduciary Assets and Liabilities
Premium payable to insurance companies			(1,384.6)
Premium deposits and credits due customers			(122.4)
Accounts payable			36.3
Fiduciary liabilities			$ 1,470.7