Notes Payable (Details) (USD $)	12 Months Ended													1 Months Ended				1 Months Ended			1 Months Ended				1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Dec. 31, 2012 5.375% notes, issued in March 2003 and due in March 2013	Dec. 31, 2011 5.375% notes, issued in March 2003 and due in March 2013	Dec. 31, 2012 5.5% notes, issued in November 2003 and due in November 2015	Dec. 31, 2011 5.5% notes, issued in November 2003 and due in November 2015	Dec. 31, 2012 5.95% notes, issued in September 2006 and due in September 2016	Dec. 31, 2011 5.95% notes, issued in September 2006 and due in September 2016	Dec. 31, 2012 5.375% notes, issued in September 2005 and due in September 2017	Dec. 31, 2011 5.375% notes, issued in September 2005 and due in September 2017	Oct. 31, 2012 2.0% notes, issued in October 2012 and due in January 2018	Dec. 31, 2012 2.0% notes, issued in October 2012 and due in January 2018	Dec. 31, 2012 6.75% notes, issued in September 2007 and due in August 2019	Dec. 31, 2011 6.75% notes, issued in September 2007 and due in August 2019	Jun. 30, 2010 5.75% notes, issued in June 2010 and due in January 2021	Dec. 31, 2012 5.75% notes, issued in June 2010 and due in January 2021	Dec. 31, 2011 5.75% notes, issued in June 2010 and due in January 2021	Oct. 31, 2012 3.25% notes, issued in October 2012 and due in October 2022	Dec. 31, 2012 3.25% notes, issued in October 2012 and due in October 2022	Dec. 31, 2012 5.875% bonds due in March 2035	Dec. 31, 2011 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	Oct. 31, 2012 Notes due in January 2018 and October 2022
Debt instrument
Notes payable	$ 2,550,000,000	$ 1,750,000,000				$ 100,000,000	$ 100,000,000	$ 150,000,000	$ 150,000,000	$ 275,000,000	$ 275,000,000	$ 175,000,000	$ 175,000,000		$ 350,000,000	$ 550,000,000	$ 550,000,000		$ 250,000,000	$ 250,000,000		$ 450,000,000	$ 250,000,000	$ 250,000,000
Interest rate (as a percent)						5.375%		5.50%		5.95%		5.375%		2.00%	2.00%	6.75%		5.75%	5.75%		3.25%	3.25%	5.875%		5.875%
Debt to total adjusted assets ratio (as a percent)					60.00%
Secured debt to total adjusted assets ratio (as a percent)					40.00%
Debt service coverage ratio				1.5
Total unencumbered assets as a percentage of outstanding unsecured debt				150.00%
Issuance of Debt														350,000,000				250,000,000			450,000,000				150,000,000	100,000,000
Percentage price paid to the investor														99.91%				99.404%			99.382%				94.578%
Effective yield (as a percent)														2.017%				5.826%			3.323%				6.318%
Net proceeds from the issuance of debt																		246,100,000							140,100,000		790,100,000
Maturity of notes and bonds payable
2013	100,000,000
2015	150,000,000
2016	275,000,000
2017	175,000,000
Thereafter	1,850,000,000
Totals	2,550,000,000
Interest incurred on notes and bonds	$ 110,400,000	$ 101,500,000	$ 89,700,000