Notes Payable (Details) (USD $)	6 Months Ended																			6 Months Ended			1 Months Ended		6 Months Ended	1 Months Ended
	Jun. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 5.5% notes, issued in November 2003 and due in November 2015	Dec. 31, 2013 5.5% notes, issued in November 2003 and due in November 2015	Jun. 30, 2014 5.95% notes, issued in September 2006 and due in September 2016	Dec. 31, 2013 5.95% notes, issued in September 2006 and due in September 2016	Jun. 30, 2014 5.375% notes, issued in September 2005 and due in September 2017	Dec. 31, 2013 5.375% notes, issued in September 2005 and due in September 2017	Jun. 30, 2014 2.0% notes, issued in October 2012 and due in January 2018	Dec. 31, 2013 2.0% notes, issued in October 2012 and due in January 2018	Jun. 30, 2014 6.75% notes, issued in September 2007 and due in August 2019	Dec. 31, 2013 6.75% notes, issued in September 2007 and due in August 2019	Jun. 30, 2014 5.75% notes, issued in June 2010 and due in January 2021	Dec. 31, 2013 5.75% notes, issued in June 2010 and due in January 2021	Jun. 30, 2014 3.25% notes, issued in October 2012 and due in October 2022	Dec. 31, 2013 3.25% notes, issued in October 2012 and due in October 2022	Jun. 30, 2014 4.65% notes, issued in July 2013 and due in August 2023	Dec. 31, 2013 4.65% notes, issued in July 2013 and due in August 2023	Jun. 30, 2014 3.875% notes, issued in June 2014 and due in July 2024	Jun. 30, 2014 5.875% bonds due in March 2035	Dec. 31, 2013 5.875% bonds due in March 2035	Jun. 30, 2011 5.875% bonds, issued in June 2011 and due in March 2035	Mar. 31, 2005 5.875% bonds, issued in March 2005 and due in March 2035	Jun. 30, 2014 Notes and bonds payable	Mar. 31, 2013 5.375% notes, issued in March 2003 and repaid in March 2013
Debt instrument
Total principal amount		$ 3,550,000,000	$ 3,200,000,000	$ 150,000,000	$ 150,000,000	$ 275,000,000	$ 275,000,000	$ 175,000,000	$ 175,000,000	$ 350,000,000	$ 350,000,000	$ 550,000,000	$ 550,000,000	$ 250,000,000	$ 250,000,000	$ 450,000,000	$ 450,000,000	$ 750,000,000	$ 750,000,000	$ 350,000,000	$ 250,000,000	$ 250,000,000
Unamortized original issuance discounts		(14,000,000)	(15,000,000)
Notes payable, net		3,535,957,000	3,185,480,000
Interest rate (as a percent)				5.50%	5.50%	5.95%	5.95%	5.375%	5.375%	2.00%	2.00%	6.75%	6.75%	5.75%	5.75%	3.25%	3.25%	4.65%	4.65%	3.875%	5.875%	5.875%				5.375%
Issuance of Debt																							150,000,000	100,000,000
Weighted average interest rate																									4.80%
Weighted average remaining years until maturity																									7 years 4 months 24 days
Repayments of Notes Payable	100,000,000																									100,000,000
Percentage price paid to the investor																				99.956%
Effective yield (as a percent)																				3.88%
Proceeds from issuance of debt	70,000,000																			346,600,000
Maturity of notes and bonds payable
2015		150,000,000
2016		275,000,000
2017		175,000,000
2018		350,000,000
Thereafter		2,600,000,000
Totals		$ 3,550,000,000