Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2021 GBP (£)	Dec. 31, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Cash and cash equivalents and short-term investments	$ 836.0			$ 1,192.0
Cash and cash equivalents and short term investments, period increase (decrease)	(356.0)
Restricted cash and investments, current	160.0			102.0
Operating funds required to be held by the Company in the U.K.	112.8		£ 84.3	60.2	£ 44.4
Increase (decrease) in operating funds required to be held by the company	45.9	£ 34.3
Premium trust balances	6,100.0			5,700.0
Fiduciary receivables	8,300.0			8,100.0
Allowance for doubtful accounts	$ 90.0			$ 98.0		$ 70.0	$ 62.0