Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2023 GBP (£)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 GBP (£)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Accounting Policies [Abstract]
Cash and cash equivalents and short-term investments	$ 1,147.0		$ 1,142.0
Cash and cash equivalents and short term investments, period increase (decrease)	5.0
Restricted cash and investments, current	120.0		115.0
Operating funds required to be held by the Company in the U.K.	80.4	£ 63.2	72.5	£ 60.1
Premium trust balances	6,900.0		6,400.0
Fiduciary receivables	9,400.0		9,500.0
Allowance for doubtful accounts	$ 79.0		$ 76.0		$ 90.0	$ 98.0