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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	6 Months Ended		12 Months Ended
	Jan. 31, 2025	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Proceeds from issuance of common stock		$ 1,378.4	$ 93.5
Money Market Accounts
Defined Benefit Plan Disclosure [Line Items]
Total cash and cash equivalents, restricted cash and fiduciary cash		15,047.9	1,947.4
Proceeds from AssuredPartners financing		13,500.0
Money Market Accounts | Interest income, premium finance revenues and other income
Defined Benefit Plan Disclosure [Line Items]
Increases dividend income		288.1
Proceeds from AssuredPartners financing		287.2
Dividend income		$ 480.1	192.0
Money Market Accounts | Over-Allotment Option
Defined Benefit Plan Disclosure [Line Items]
Proceeds from issuance of common stock	$ 1,280.0
Qualified Contributory Savings and Thrift 401(k) Plan
Defined Benefit Plan Disclosure [Line Items]
Matching contributions by employer, percentage		100.00%
Percentage of eligible compensation for matching contributions by employer		5.00%
Matching contributions vesting schedule		5 years
Contribution expense to plan		$ 59.8	$ 56.0
Employer match on eligible compensation				5.00%	5.00%

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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-Topic 220
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