Cash and Cash Equivalents and Short-Term Investments (Details) £ in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2023 GBP (£)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 GBP (£)
Cash, Cash Equivalents, and Short-Term Investments [Abstract]
Cash and cash equivalents and short-term investments	$ 983.0		$ 1,142.0
Decrease in cash and cash equivalents and short term investments	(159.0)
Restricted cash and investments	115.0		115.0
Operating funds required to be held in U.K.	$ 75.5	£ 62.1	$ 72.5	£ 60.1