Notes Payable - General (Details) £ in Thousands	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Jul. 31, 2020 USD ($)	May 31, 2020 USD ($)	Jan. 31, 2020	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Jun. 30, 2019 USD ($)	May 31, 2019 GBP (£)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jun. 30, 2011 USD ($)	Mar. 31, 2005 USD ($)
3.250% notes, issued in May 2020 and due in January 2031
Debt
Face amount of notes				$ 600,000,000
3.250% notes, issued in July 2020 and due in January 2031
Debt
Face amount of notes			$ 350,000,000
Notes and bonds payable
Debt
Total principal amount	$ 7,007,000,000					$ 6,318,000,000
Unamortized net original issuance premiums and deferred financing costs	(12,000,000)					(30,000,000)
Net payable amount	6,995,000,000					6,288,000,000
Notes and bonds payable | 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%	5.75%
Notes and bonds payable | 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%
Notes and bonds payable | 3.250% notes, issued in October 2012 and due in October 2022
Debt
Face amount of notes													$ 450,000,000
Notes and bonds payable | 3.250% notes, issued in December 2017 and due in October 2022
Debt
Face amount of notes										$ 500,000,000
Notes and bonds payable | 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%
Notes and bonds payable | 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%
Notes and bonds payable | 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%
Notes and bonds payable | 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%
Notes and bonds payable | 4.125% notes, issued in September 2014 and due in October 2026
Debt
Face amount of notes												$ 250,000,000
Notes and bonds payable | 4.125% notes, issued in March 2017 and due in October 2026
Debt
Face amount of notes											$ 400,000,000
Notes and bonds payable | 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%
Notes and bonds payable | 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%
Notes and bonds payable | 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								$ 500,000,000.0
Notes and bonds payable | 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%
Notes and bonds payable | 3.250% notes, issued in May 2020 and due in January 2031
Debt
Face amount of notes				$ 600,000,000.0
Notes and bonds payable | 3.250% notes, issued in July 2020 and due in January 2031
Debt
Interest rate (as a percent)			3.25%
Face amount of notes			$ 350,000,000.0
Notes and bonds payable | 2.730% notes, issued in May 2019 and due in May 2034
Debt
Total principal amount	$ 407,000,000	£ 315,000				418,000,000	£ 315,000
Interest rate (as a percent)	2.73%	2.73%							2.73%
Face amount of notes | £									£ 315,000
Notes and bonds payable | 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%
Notes and bonds payable | 5.875% bonds, issued in March 2005 and due in March 2035
Debt
Face amount of notes															$ 100,000,000
Notes and bonds payable | 5.875% bonds, issued in June 2011 and due in March 2035
Debt
Face amount of notes														$ 150,000,000
Notes and bonds payable | 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%
Notes and bonds payable | 4.650% notes, issued in March 2017 and due in March 2047
Debt
Face amount of notes											$ 300,000,000
Notes and bonds payable | 4.650% notes, issued in December 2017 and due in March 2047
Debt
Face amount of notes										$ 250,000,000