Senior Unsecured Notes and Secured Debt (Details) (USD $)	3 Months Ended	9 Months Ended				9 Months Ended		3 Months Ended				3 Months Ended	9 Months Ended
	Sep. 30, 2010	Sep. 30, 2011 Year Property State	Sep. 30, 2010 Activity Property	Dec. 31, 2010		Sep. 30, 2011 Senior Unsecured Notes [Member]	Sep. 30, 2011 Secured Debt [Member]	Jun. 30, 2010 Senior unsecured convertible notes due 2020 [Member]	Sep. 30, 2010 Senior unsecured convertible notes due 2017 [Member]	Mar. 31, 2011 Senior unsecured notes due 2016 [Member]	Mar. 31, 2011 Senior unsecured notes due 2022 [Member]	Mar. 31, 2011 Senior unsecured notes due 2041 [Member]	Sep. 30, 2011 First mortgage loans [Member] Property
Principal payments due on debt obligations
2011		$ 7,522,000				$ 0	$ 7,522,000
2012		182,846,000				76,853,000	105,993,000
2013		575,041,000				300,000,000	275,041,000
2014		186,726,000				0	186,726,000
2015		431,280,000				250,000,000	181,280,000
Thereafter		4,949,212,000				3,838,077,000	1,111,135,000
Totals		6,332,627,000				4,464,930,000	1,867,697,000
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes		4,432,092,000		3,034,949,000	[1]	4,432,092,000
Notes, annual stated interest rates, Minimum						0.0300	0.0460
Notes, annual stated interest rates, Maximum						0.0800	0.1000
Par value of senior debt						4,464,930,000	1,867,697,000
Senior unsecured convertible notes issued								450,000,000	450,000,000	400,000,000	600,000,000	400,000,000
Senior unsecured convertible notes, interest rate								6.125%	4.70%	3.625%	5.25%	6.50%
Proceeds from issuance of senior unsecured notes		1,381,086,000	1,378,180,000					446,328,000	445,768,000			1,381,086,000
Other income less than (in excess of) cash received	(9,099,000)	0	(34,171,000)
Equity component of convertible debt, reacquire			(9,689,000)
Assumed first mortgage loans secured			363,515,000										693,785,000
First mortgage loans assumed		58,470,000
First mortgage loans interest		5.80%	6.40%										5.40%
Number of properties secured by mortgages		32	41										36
Additional Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Secured debt		1,888,083,000		1,125,906,000	[1]
Carrying values of properties securing the debt		$ 3,534,058,000

[1] NOTE: The consolidated balance sheet at December 31, 2010 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.