Notes Payable (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Debt instrument
Total principal amount	$ 3,800,000,000	$ 3,800,000,000
Unamortized original issuance discounts	(14,000,000)	(15,000,000)
Notes payable, net	3,785,718,000	3,785,372,000
Maturity of notes and bonds payable
2015	150,000,000
2016	275,000,000
2017	175,000,000
2018	350,000,000
2019	550,000,000
Thereafter	2,300,000,000
Totals	3,800,000,000
5.5% notes, issued in November 2003 and due in November 2015
Debt instrument
Total principal amount	150,000,000	150,000,000
Interest rate (as a percent)	5.50%	5.50%
5.95% notes, issued in September 2006 and due in September 2016
Debt instrument
Total principal amount	275,000,000	275,000,000
Interest rate (as a percent)	5.95%	5.95%
5.375% notes, issued in September 2005 and due in September 2017
Debt instrument
Total principal amount	175,000,000	175,000,000
Interest rate (as a percent)	5.375%	5.375%
2.0% notes, issued in October 2012 and due in January 2018
Debt instrument
Total principal amount	350,000,000	350,000,000
Interest rate (as a percent)	2.00%	2.00%
6.75% notes, issued in September 2007 and due in August 2019
Debt instrument
Total principal amount	550,000,000	550,000,000
Interest rate (as a percent)	6.75%	6.75%
5.75% notes, issued in June 2010 and due in January 2021
Debt instrument
Total principal amount	250,000,000	250,000,000
Interest rate (as a percent)	5.75%	5.75%
3.25% notes, issued in October 2012 and due in October 2022
Debt instrument
Total principal amount	450,000,000	450,000,000
Interest rate (as a percent)	3.25%	3.25%
4.65% notes, issued in July 2013 and due in August 2023
Debt instrument
Total principal amount	750,000,000	750,000,000
Interest rate (as a percent)	4.65%	4.65%
3.875% notes, issued in June 2014 and due in July 2024
Debt instrument
Total principal amount	350,000,000	350,000,000
Interest rate (as a percent)	3.875%	3.875%
4.125% notes, issued in September 2014 and due in October 2026
Debt instrument
Total principal amount	250,000,000	250,000,000
Interest rate (as a percent)	4.125%	4.125%
5.875% bonds due in March 2035
Debt instrument
Total principal amount	250,000,000	250,000,000
Interest rate (as a percent)	5.875%	5.875%
5.875% bonds, issued in March 2005 and due in March 2035
Debt instrument
Issuance of Debt	100,000,000	100,000,000
5.875% bonds, issued in June 2011 and due in March 2035
Debt instrument
Issuance of Debt	$ 150,000,000	$ 150,000,000
Notes and bonds payable
Debt instrument
Weighted average interest rate	4.80%
Weighted average remaining years until maturity	7 years