Notes Payable - General (Details) - Notes and bonds payable	Sep. 30, 2019 GBP (£)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	May 31, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	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		$ 6,287,000,000			$ 5,400,000,000
Unamortized net original issuance premiums and deferred financing costs		(31,000,000)			(23,000,000)
Net payable amount		6,256,000,000			5,377,000,000
5.750% notes, issued in June 2010 and due in January 2021
Debt
Total principal amount		$ 250,000,000			$ 250,000,000
Interest rate (as a percent)	5.75%	5.75%			5.75%
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%			3.25%
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%			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%
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%	3.875%
Face amount of notes						$ 500,000,000
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%
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%			3.00%
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%			3.65%
3.250% notes, issued in June 2019 and due in June 2029
Debt
Total principal amount		$ 500,000,000			$ 0
Interest rate (as a percent)	3.25%	3.25%	3.25%
Face amount of notes			$ 500,000,000
2.730% notes, issued in May 2019 and due in May 2034
Debt
Total principal amount	£ 315,000,000.0	$ 387,000,000			0
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%
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%			4.65%
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