Notes Payable - Narrative (Details) - Notes payable $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Mar. 31, 2023 GBP (£)	Mar. 31, 2022 GBP (£)	Jan. 31, 2022 GBP (£)	Jul. 31, 2021 GBP (£)	Oct. 31, 2020 GBP (£)
Debt
Weighted average interest rate (as a percent)			3.40%
Weighted average remaining years until maturity	6 years 9 months 18 days
Interest incurred on notes and bonds | $	$ 130.3	$ 103.1
1.625% Notes due 2030
Debt
Principal amount			£ 400,000,000				£ 400,000,000
Interest rate (as a percent)			1.625%				0.01625%
1.125% Notes due 2027
Debt
Principal amount			£ 400,000,000			£ 400,000,000
Interest rate (as a percent)			1.125%			0.01125%
1.750% Notes due 2033
Debt
Principal amount			£ 350,000,000			£ 350,000,000
Interest rate (as a percent)			1.75%			0.0175%
1.875% Notes due 2027
Debt
Principal amount			£ 250,000,000	£ 250,000,000.0	£ 250,000,000
Interest rate (as a percent)			1.875%	1.875%	0.01875%
2.500% Notes due 2042
Debt
Principal amount			£ 250,000,000	£ 250,000,000.0	£ 250,000,000
Interest rate (as a percent)			2.50%	2.50%	0.025%
Maximum
Debt
Debt to total adjusted assets ratio (as a percent)	60.00%
Secured debt to total adjusted assets ratio (as a percent)	40.00%
Minimum
Debt
Debt service coverage ratio	1.5
Total unencumbered assets as a percentage of outstanding unsecured debt	150.00%