Change in Presentation of Fiduciary Assets and Liabilities - Summary of Changes to Presentation of Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents		$ 738.4
Restricted cash	$ 190.6	199.0	$ 136.9
Fiduciary assets	26,907.9	18,236.7
Accounts receivable, net	3,786.6	2,911.1
Other current assets	450.1	399.0
Total current assets	32,116.1	22,285.2
Total assets	51,615.8	38,358.4	$ 33,236.1
Fiduciary liabilities	26,907.9	18,236.7
Accrued compensation and other accrued liabilities	2,553.1	2,003.3
Total current liabilities	31,064.7	21,338.6
Total liabilities	40,800.5	29,168.2
Total liabilities and stockholders' equity	$ 51,615.8	38,358.4
Reported 2022
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents		342.3
Restricted cash		4,621.9
Premiums and fees receivable		16,408.9
Other current assets		1,461.5
Total current assets		22,834.6
Total assets		38,907.8
Premiums payable to underwriting enterprises		18,698.2
Accrued compensation and other accrued liabilities		2,091.2
Total current liabilities		21,888.0
Total liabilities		29,717.6
Total liabilities and stockholders' equity		38,907.8
Change
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents		396.1
Restricted cash		(4,621.9)
Premiums and fees receivable		(16,408.9)
Fiduciary assets		18,236.7
Accounts receivable, net		2,911.1
Other current assets		(1,062.5)
Total current assets		(549.4)
Total assets		(549.4)
Premiums payable to underwriting enterprises		(18,698.2)
Fiduciary liabilities		18,236.7
Accrued compensation and other accrued liabilities		(87.9)
Total current liabilities		(549.4)
Total liabilities		(549.4)
Total liabilities and stockholders' equity		$ (549.4)