Notes Payable - Note Issuances (Details) - Notes payable	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Jul. 31, 2021 GBP (£)	Jul. 31, 2020 USD ($)	May 31, 2020 USD ($)
1.125% notes, issued in July 2021 and due in July 2027
Debt
Stated interest rate (as a percent)	1.125%	1.125%	1.125%
Principal amount | £		£ 400,000,000	£ 400,000,000
Price of par value (as a percent)			99.31%
Effective yield (as a percent)			1.24%
1.750% notes, issued in July 2021 and due in July 2033
Debt
Stated interest rate (as a percent)	1.75%	1.75%	1.75%
Principal amount | £		£ 350,000,000	£ 350,000,000
Price of par value (as a percent)			99.84%
Effective yield (as a percent)			1.76%
3.250% notes, issued in May 2020 and due in January 2031
Debt
Stated interest rate (as a percent)					3.25%
Principal amount | $	$ 600,000,000				$ 600,000,000
Price of par value (as a percent)					98.99%
Effective yield (as a percent)					3.36%
3.250% notes, issued in July 2020 and due in January 2031
Debt
Stated interest rate (as a percent)				3.25%
Principal amount | $	$ 350,000,000			$ 350,000,000
Price of par value (as a percent)				108.24%
Effective yield (as a percent)				2.34%