Notes Payable - Note Issuances Narrative (Details)	1 Months Ended			9 Months Ended
	Jul. 31, 2021 USD ($)	Jul. 31, 2020 USD ($)	May 31, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 GBP (£)	Jul. 31, 2021 GBP (£)
Debt
Net proceeds				$ 1,033,387,000	$ 972,766,000
Notes payable | 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
Net proceeds	$ 546,300,000
Notes payable | 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
Net proceeds	$ 480,600,000
Notes payable | 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
Net proceeds		$ 376,600,000
Notes payable | 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
Net proceeds			$ 590,000,000