Notes Payable - General (Details) - Notes and bonds payable	1 Months Ended
	Jan. 31, 2021 USD ($)	Jan. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Oct. 31, 2020 GBP (£)	Jul. 31, 2020 USD ($)	May 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	May 31, 2019 GBP (£)	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jun. 30, 2011 USD ($)	Mar. 31, 2005 USD ($)
Debt
Total principal amount			$ 8,303,000,000					$ 6,318,000,000
Unamortized net original issuance premiums and deferred financing costs			(35,000,000)					(30,000,000)
Net payable amount			8,268,000,000					6,288,000,000
5.750% notes, issued in June 2010 and due in January 2021
Debt
Total principal amount			0					$ 250,000,000
Interest rate (as a percent)		5.75%						5.75%
Early repayment of notes payable		$ 250,000,000.0
3.250% notes, $450 issued in October 2012 and $500 issued in December 2017, both due in October 2022
Debt
Total principal amount			$ 950,000,000					$ 950,000,000
Interest rate (as a percent)			3.25%	3.25%
3.250% notes, $450 issued in October 2012 and $500 issued in December 2017, both due in October 2022 | Subsequent event
Debt
Interest rate (as a percent)	3.25%
Early repayment of notes payable	$ 950,000,000.0
3.250% notes, issued in October 2012 and due in October 2022
Debt
Face amount of notes															$ 450,000,000
3.250% notes, issued in December 2017 and due in October 2022
Debt
Face amount of notes												$ 500,000,000
4.650% notes, issued in July 2013 and due in August 2023
Debt
Total principal amount			$ 750,000,000					750,000,000
Interest rate (as a percent)			4.65%	4.65%
3.875% notes, issued in June 2014 and due in July 2024
Debt
Total principal amount			$ 350,000,000					350,000,000
Interest rate (as a percent)			3.875%	3.875%
3.875% notes, issued in April 2018 and due in April 2025
Debt
Total principal amount			$ 500,000,000					500,000,000
Interest rate (as a percent)			3.875%	3.875%							3.875%
Face amount of notes											$ 500,000,000
0.750% notes, issued in December 2020 and due in March 2026
Debt
Total principal amount			$ 325,000,000					0
Interest rate (as a percent)			0.75%	0.75%
Face amount of notes			$ 325,000,000.0
4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026
Debt
Total principal amount			$ 650,000,000					650,000,000
Interest rate (as a percent)			4.125%	4.125%
4.125% notes, issued in September 2014 and due in October 2026
Debt
Face amount of notes														$ 250,000,000
4.125% notes, issued in March 2017 and due in October 2026
Debt
Face amount of notes													$ 400,000,000
3.000% notes, issued in October 2016 and due in January 2027
Debt
Total principal amount			$ 600,000,000					600,000,000
Interest rate (as a percent)			3.00%	3.00%
3.650% notes, issued in December 2017 and due in January 2028
Debt
Total principal amount			$ 550,000,000					550,000,000
Interest rate (as a percent)			3.65%	3.65%
3.250% notes, issued in June 2019 and due in June 2029
Debt
Total principal amount			$ 500,000,000					500,000,000
Interest rate (as a percent)			3.25%	3.25%					3.25%
Face amount of notes									$ 500,000,000
1.625% notes, issued in October 2020 and due December 2030
Debt
Total principal amount			$ 547,000,000	£ 400,000,000.0				0
Interest rate (as a percent)			1.625%	1.625%	1.625%
Face amount of notes | £					£ 400,000,000
3.250% notes, $600 issued in May 2020 and $350 issued in July 2020, both due in January 2031
Debt
Total principal amount			$ 950,000,000					0
Interest rate (as a percent)			3.25%	3.25%
3.250% notes, issued in May 2020 and due in January 2031
Debt
Interest rate (as a percent)							3.25%
Face amount of notes							$ 600,000,000
3.250% notes, issued in July 2020 and due in January 2031
Debt
Interest rate (as a percent)						3.25%
Face amount of notes						$ 350,000,000
1.800% notes, issued in December 2020 and due in March 2033
Debt
Total principal amount			$ 400,000,000					0
Interest rate (as a percent)			1.80%	1.80%
Face amount of notes			$ 400,000,000.0
2.730% notes, issued in May 2019 and due in May 2034
Debt
Total principal amount			$ 431,000,000	£ 315,000,000.0				418,000,000
Interest rate (as a percent)			2.73%	2.73%						2.73%
Face amount of notes | £										£ 315,000,000
5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035
Debt
Total principal amount			$ 250,000,000					250,000,000
Interest rate (as a percent)			5.875%	5.875%
5.875% bonds, issued in March 2005 and due in March 2035
Debt
Face amount of notes																	$ 100,000,000
5.875% bonds, issued in June 2011 and due in March 2035
Debt
Face amount of notes																$ 150,000,000
4.650% notes, $300 issued in March 2017 and $250 issued in December 2017, both due in March 2047
Debt
Total principal amount			$ 550,000,000					$ 550,000,000
Interest rate (as a percent)			4.65%	4.65%
4.650% notes, issued in March 2017 and due in March 2047
Debt
Face amount of notes													$ 300,000,000
4.650% notes, issued in December 2017 and due in March 2047
Debt
Face amount of notes												$ 250,000,000