Notes Payable (Details) (USD $)	9 Months Ended		1 Months Ended																	1 Months Ended				1 Months Ended			1 Months Ended			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Mar. 31, 2013 5.375% notes, issued in March 2003 and repaid in March 2013	Sep. 30, 2013 5.375% notes, issued in March 2003 and repaid in March 2013	Dec. 31, 2012 5.375% notes, issued in March 2003 and repaid in March 2013	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Jul. 31, 2013 4.65% notes, issued in July 2013 and due in August 2023	Sep. 30, 2013 4.65% notes, issued in July 2013 and due in August 2023	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2013 Notes and bonds payable
Debt instrument
Notes payable	$ 3,200,000,000	$ 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		$ 750,000,000	$ 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%		4.65%	5.875%	5.875%		5.875%	5.875%		5.875%	5.875%
Effective yield (as a percent)																				4.678%
Weighted average interest rate																														4.90%
Weighted average remaining years until maturity																														7 years 10 months 24 days
Issuance of Debt																								150,000,000			100,000,000
Repayments of Notes Payable	100,000,000		100,000,000
Percentage price paid to the investor																				99.775%
Proceeds from issuance of debt	$ 70,000,000																			$ 741,400,000