Notes Payable (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt instrument
Total principal amount				$ 3,650,000	$ 3,800,000
Unamortized original issuance discounts				(13,000)	(15,000)
Notes payable, net				3,636,746	$ 3,785,372
Principal payments on mortgages payable				150,000		$ 100,000
Proceeds from Issuance of Debt				250,000		70,000
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
Minimum
Debt instrument
Interest rate (as a percent)				2.00%	2.00%
Effective yield (as a percent)				2.20%	2.20%
Maximum
Debt instrument
Interest rate (as a percent)				6.90%	6.90%
Effective yield (as a percent)				8.90%	9.00%
Class E Cumulative Redeemable Preferred Stock
Debt instrument
Preferred stock, dividend rate (as a percent)		6.75%
5.5% notes, issued in November 2003 and due in November 2015
Debt instrument
Total principal amount					$ 150,000
Interest rate (as a percent)	5.50%				5.50%
Principal payments on mortgages payable	$ 150,000
5.95% 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%
5.375% 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%
2.0% 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%
6.75% 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%
5.75% 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%
3.25% 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%
4.65% 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	$ 350,000
Interest rate (as a percent)			3.875%	3.875%	3.875%
Percentage price paid to the investor			99.956%
Effective yield (as a percent)			3.88%
Proceeds from Issuance of Debt			$ 346,700
4.125% notes, issued in September 2014 and due in October 2026
Debt instrument
Total principal amount		$ 250,000		$ 250,000	$ 250,000
Interest rate (as a percent)		4.125%		4.125%	4.125%
Percentage price paid to the investor		99.499%
Effective yield (as a percent)		4.178%
Proceeds from Issuance of Debt		$ 246,400
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 June 2011 and due in March 2035
Debt instrument
Total principal amount				$ 150	$ 150
5.875% bonds, issued in March 2005 and due in March 2035
Debt instrument
Total principal amount				$ 100	100
Notes and bonds payable
Debt instrument
Weighted average interest rate				4.70%
Weighted average remaining years until maturity				6 years 6 months
Interest incurred on notes and bonds				$ 179,500	$ 166,500	$ 138,900
Notes and bonds payable | Minimum
Debt instrument
Debt service coverage ratio				1.5
Total unencumbered assets as a percentage of outstanding unsecured debt				150.00%
Notes and bonds payable | Maximum
Debt instrument
Debt to total adjusted assets ratio (as a percent)				60.00%
Secured debt to total adjusted assets ratio (as a percent)				40.00%