Cash and Cash Equivalents and Short-term Investments (Details) £ in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents and short-term investments	$ 919.0		$ 919.0		$ 1,192.0
Cash and cash equivalents and short term investments, period increase (decrease)			(273.0)
Restricted cash and investments	162.0		162.0		102.0
Operating funds required to be held in U.K.	115.3		$ 115.3	£ 84.3	$ 60.2	£ 44.4
Increase (decrease) in operating funds required to be held by the Company	$ 46.5	£ 34.0