Notes Payable - General Narrative (Details) - Notes and bonds payable $ in Millions	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Jul. 31, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Oct. 31, 2020 GBP (£)
Debt
Unamortized net original issuance premiums | $	$ 7.2			$ 14.6
Deferred financing costs | $	$ 51.0			$ 49.2
Weighted average interest rate (as a percent)	3.20%	3.20%
Weighted average remaining years until maturity	8 years 3 months 18 days
1.625% notes, issued in October 2020 and due in December 2030
Debt
Principal amount		£ 400,000,000			£ 400,000,000
Stated interest rate (as a percent)	1.625%	1.625%			1.625%
1.125% notes, issued in July 2021 and due in July 2027
Debt
Principal amount		£ 400,000,000	£ 400,000,000
Stated interest rate (as a percent)	1.125%	1.125%	1.125%
1.750% notes, issued in July 2021 and due in July 2033
Debt
Principal amount		£ 350,000,000	£ 350,000,000
Stated interest rate (as a percent)	1.75%	1.75%	1.75%