Notes Payable (Details) (USD $)	6 Months Ended		6 Months Ended																		1 Months Ended			1 Months Ended			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 5.375% notes, issued in March 2003 and due in March 2013	Dec. 31, 2012 5.375% notes, issued in March 2003 and due in March 2013	Jun. 30, 2013 5.5% notes, issued in November 2003 and due in November 2015	Dec. 31, 2012 5.5% notes, issued in November 2003 and due in November 2015	Jun. 30, 2013 5.95% notes, issued in September 2006 and due in September 2016	Dec. 31, 2012 5.95% notes, issued in September 2006 and due in September 2016	Jun. 30, 2013 5.375% notes, issued in September 2005 and due in September 2017	Dec. 31, 2012 5.375% notes, issued in September 2005 and due in September 2017	Jun. 30, 2013 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	Jun. 30, 2013 6.75% notes, issued in September 2007 and due in August 2019	Dec. 31, 2012 6.75% notes, issued in September 2007 and due in August 2019	Jun. 30, 2013 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	Jun. 30, 2013 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	Jun. 30, 2013 5.875% bonds due in March 2035	Dec. 31, 2012 5.875% bonds due in March 2035	Jun. 30, 2011 5.875% bonds, issued in June 2011 and due in March 2035	Jun. 30, 2013 5.875% bonds, issued in June 2011 and due in March 2035	Dec. 31, 2012 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, 2013 5.875% bonds, issued in March 2005 and due in March 2035	Dec. 31, 2012 5.875% bonds, issued in March 2005 and due in March 2035	Jun. 30, 2013 Notes and bonds payable
Debt instrument
Notes payable	$ 2,450,000,000	$ 2,550,000,000		$ 100,000,000	$ 150,000,000	$ 150,000,000	$ 275,000,000	$ 275,000,000	$ 175,000,000	$ 175,000,000	$ 350,000,000	$ 350,000,000	$ 550,000,000	$ 550,000,000	$ 250,000,000	$ 250,000,000	$ 450,000,000	$ 450,000,000	$ 250,000,000	$ 250,000,000
Interest rate (as a percent)			5.375%	5.375%	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%	5.875%	5.875%		5.875%	5.875%		5.875%	5.875%
Weighted average interest rate (as a percent)																											4.97%
Weighted average remaining years until maturity																											7 years 6 months
Repayment price as a percentage of the principal amount, plus accrued and unpaid interest			100.00%
Issuance of Debt																					150,000,000			100,000,000
Repayments of Notes Payable	$ 100,000,000		$ 100,000,000