Notes Payable - Note Issuances (Details) - Notes Payable	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
2.730% notes, issued in May 2019 and due in May 2034
Debt
Stated interest rate (as a percent)	2.73%	2.73%
Principal amount issued | £	£ 315,000,000
Price of par value (as a percent)	100.00%	100.00%
Effective yield (as a percent)	2.73%	2.73%
3.250% notes, issued in June 2019 and due in June 2029
Debt
Stated interest rate (as a percent)	3.25%	3.25%
Principal amount issued		$ 500,000,000
Price of par value (as a percent)	99.36%	99.36%
Effective yield (as a percent)	3.33%	3.33%
3.875% notes, issued in April 2018 and due in April 2025
Debt
Stated interest rate (as a percent)	3.875%	3.875%	3.875%
Principal amount issued			$ 500,000,000
Price of par value (as a percent)			99.50%
Effective yield (as a percent)			3.96%
4.125% notes, issued in March 2017 and due in October 2026
Debt
Stated interest rate (as a percent)				4.125%
Principal amount issued				$ 400,000,000	$ 400,000,000
Price of par value (as a percent)				102.98%
Effective yield (as a percent)				3.75%
4.650% notes, issued in March 2017 and due in March 2047
Debt
Stated interest rate (as a percent)				4.65%
Principal amount issued				$ 300,000,000	$ 300,000,000
Price of par value (as a percent)				99.97%
Effective yield (as a percent)				4.65%
3.250% notes, issued in December 2017 and due in October 2022
Debt
Stated interest rate (as a percent)				3.25%
Principal amount issued				$ 500,000,000
Price of par value (as a percent)				101.77%
Effective yield (as a percent)				2.84%
3.650% notes, issued in December 2017 and due in January 2028
Debt
Stated interest rate (as a percent)				3.65%
Principal amount issued				$ 550,000,000
Price of par value (as a percent)				99.78%
Effective yield (as a percent)				3.68%
4.650% notes, issued in December 2017 and due in March 2047
Debt
Stated interest rate (as a percent)				4.65%
Principal amount issued				$ 250,000,000
Price of par value (as a percent)				105.43%
Effective yield (as a percent)				4.32%