Notes Payable - General (Details) - USD ($)	1 Months Ended						12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Sep. 30, 2014	Oct. 31, 2012	Jun. 30, 2011	Mar. 31, 2005	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2018
Debt
Notes payable, net	$ 5,230,244,000						$ 5,376,797,000	$ 5,230,244,000
Notes Payable
Debt
Notes payable, gross	5,250,000,000						5,400,000,000	5,250,000,000
Unamortized net original issuance premiums and deferred financing costs	(20,000,000)						(23,000,000)	(20,000,000)
Notes payable, net	$ 5,230,000,000						$ 5,377,000,000	$ 5,230,000,000
Notes Payable | 2.000% notes, issued in October 2012 and due in January 2018
Debt
Interest rate (as a percent)	2.00%						2.00%	2.00%		2.00%
Notes payable, gross	$ 350,000,000						$ 0	$ 350,000,000
Notes Payable | 5.750% notes, issued in June 2010 and due in January 2021
Debt
Interest rate (as a percent)	5.75%						5.75%	5.75%
Notes payable, gross	$ 250,000,000						$ 250,000,000	$ 250,000,000
Notes Payable | 3.250% notes, $450 issued in October 2012 and $500 issued in December 2017, both due in October 2022
Debt
Interest rate (as a percent)	3.25%						3.25%	3.25%
Notes payable, gross	$ 950,000,000						$ 950,000,000	$ 950,000,000
Notes Payable | 3.250% notes, issued in October 2012 and due in October 2022
Debt
Issuance of debt				$ 450,000,000
Notes Payable | 3.250% notes, issued in December 2017 and due in October 2022
Debt
Interest rate (as a percent)	3.25%							3.25%
Issuance of debt	$ 500,000,000							$ 500,000,000
Notes Payable | 4.650% notes, issued in July 2013 and due in August 2023
Debt
Interest rate (as a percent)	4.65%						4.65%	4.65%
Notes payable, gross	$ 750,000,000						$ 750,000,000	$ 750,000,000
Notes Payable | 3.875% notes, issued in June 2014 and due in July 2024
Debt
Interest rate (as a percent)	3.875%						3.875%	3.875%
Notes payable, gross	$ 350,000,000						$ 350,000,000	$ 350,000,000
Notes Payable | 3.875% notes, issued in April 2018 and due in April 2025
Debt
Interest rate (as a percent)							3.875%
Notes payable, gross	$ 0						$ 500,000,000	$ 0
Issuance of debt							$ 500,000,000
Notes Payable | 4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026
Debt
Interest rate (as a percent)	4.125%						4.125%	4.125%
Notes payable, gross	$ 650,000,000						$ 650,000,000	$ 650,000,000
Notes Payable | 4.125% notes, issued in September 2014 and due in October 2026
Debt
Issuance of debt			$ 250,000,000
Notes Payable | 4.125% notes, issued in March 2017 and due in October 2026
Debt
Interest rate (as a percent)	4.125%							4.125%
Issuance of debt		$ 400,000,000						$ 400,000,000
Notes Payable | 3.000% notes, issued in October 2016 and due in January 2027
Debt
Interest rate (as a percent)	3.00%						3.00%	3.00%	3.00%
Notes payable, gross	$ 600,000,000						$ 600,000,000	$ 600,000,000
Issuance of debt									$ 600,000,000
Notes Payable | 3.650% notes, issued in December 2017 and due in January 2028
Debt
Interest rate (as a percent)	3.65%						3.65%	3.65%
Notes payable, gross	$ 550,000,000						$ 550,000,000	$ 550,000,000
Issuance of debt								$ 550,000,000
Notes Payable | 5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035
Debt
Interest rate (as a percent)	5.875%						5.875%	5.875%
Notes payable, gross	$ 250,000,000						$ 250,000,000	$ 250,000,000
Notes Payable | 5.875% bonds, issued in March 2005 and due in March 2035
Debt
Issuance of debt						$ 100,000,000
Notes Payable | 5.875% bonds, issued in June 2011 and due in March 2035
Debt
Issuance of debt					$ 150,000,000
Notes Payable | 4.650% notes, $300 issued in March 2017 and $250 issued in December 2017, both due in March 2047
Debt
Interest rate (as a percent)							4.65%
Notes payable, gross	$ 550,000,000						$ 550,000,000	$ 550,000,000
Notes Payable | 4.650% notes, issued in March 2017 and due in March 2047
Debt
Interest rate (as a percent)	4.65%							4.65%
Issuance of debt		$ 300,000,000						$ 300,000,000
Notes Payable | 4.650% notes, issued in December 2017 and due in March 2047
Debt
Interest rate (as a percent)	4.65%							4.65%
Issuance of debt	$ 250,000,000							$ 250,000,000