Notes Payable (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended	12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Debt instrument
Total principal amount		$ 3,650,000	$ 3,650,000
Unamortized original issuance discounts and deferred financing costs		(30,000)	(32,000)
Notes payable, net		3,620,275	3,617,973
Maturity of notes and bonds payable
2016		275,000
2017		175,000
2018		350,000
2019		550,000
Thereafter		2,300,000
Totals		3,650,000
5.950% notes, issued in September 2006 and due in September 2016
Debt instrument
Total principal amount		$ 275,000	$ 275,000
Interest rate (as a percent)		5.95%	5.95%
5.950% notes, issued in September 2006 and due in September 2016 | Forecast
Debt instrument
Interest rate (as a percent)	5.95%
Long term debt notes payable	$ 275,000
5.375% notes, issued in September 2005 and due in September 2017
Debt instrument
Total principal amount		$ 175,000	$ 175,000
Interest rate (as a percent)		5.375%	5.375%
2.000% notes, issued in October 2012 and due in January 2018
Debt instrument
Total principal amount		$ 350,000	$ 350,000
Interest rate (as a percent)		2.00%	2.00%
6.750% notes, issued in September 2007 and due in August 2019
Debt instrument
Total principal amount		$ 550,000	$ 550,000
Interest rate (as a percent)		6.75%	6.75%
5.750% notes, issued in June 2010 and due in January 2021
Debt instrument
Total principal amount		$ 250,000	$ 250,000
Interest rate (as a percent)		5.75%	5.75%
3.250% notes, issued in October 2012 and due in October 2022
Debt instrument
Total principal amount		$ 450,000	$ 450,000
Interest rate (as a percent)		3.25%	3.25%
4.650% notes, issued in July 2013 and due in August 2023
Debt instrument
Total principal amount		$ 750,000	$ 750,000
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	$ 350,000
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	$ 250,000
Interest rate (as a percent)		4.125%	4.125%
5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035
Debt instrument
Total principal amount		$ 250,000	$ 250,000
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	$ 100,000
5.875% bonds, issued in June 2011 and due in March 2035
Debt instrument
Issuance of Debt		$ 150,000	$ 150,000
Notes and bonds payable
Debt instrument
Weighted average interest rate		4.70%
Weighted average remaining years until maturity		6 years