Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended		12 Months Ended
	Dec. 31, 2023 USD ($)	Sep. 30, 2023	Dec. 31, 2023 USD ($) Segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments | Segment			4
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, adoption date	Feb. 10, 2023		Feb. 10, 2023
Change in accounting principle, accounting standards update, immaterial effect	true		true
Change in accounting principle, accounting standards update, description			ASU 2020-04
Gains recognized on sales of businesses			$ 143,300,000	$ 4,500,000	$ 9,600,000
Average Annual EBITDAC, operating profit earned period maximum			3 years
Accumulated impairment losses	$ 0		$ 0
Goodwill impairment loss			0	0	0
Gain on Disposition of Assets			143,300,000	4,500,000	$ 9,600,000
Carrying value of fixed-rate borrowings	2,739,600,000		2,739,600,000	2,738,800,000
Long-term Debt, Fair Value	2,487,400,000		2,487,400,000	$ 2,355,700,000
Revolving and term loan			1,075,000,000
Amount written off as uncollectable from reinsurance carriers			$ 0
Third-party Claims Administration and Adjusting Services Businesses | Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Gains recognized on sales of businesses	$ 134,600,000
Percentage of business sold		50.00%
Minimum
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	3 years		3 years
Finite-Lived Intangible Asset, Useful Life	3 years		3 years
Maximum
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	40 years		40 years
Finite-Lived Intangible Asset, Useful Life	15 years		15 years