Senior Unsecured Notes and Secured Debt (Details) (USD $)	3 Months Ended	6 Months Ended				6 Months Ended		3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			6 Months Ended		3 Months Ended				3 Months Ended	6 Months Ended
	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010		Jun. 30, 2011 Senior Unsecured Notes [Member]	Jun. 30, 2011 Secured Debt [Member]	Mar. 31, 2009 Senior unsecured convertible notes due 2026 [Member]	Jun. 30, 2011 Senior unsecured convertible notes due 2026 [Member]	Jun. 30, 2010 Senior unsecured convertible notes due 2026 [Member]	Dec. 31, 2006 Senior unsecured convertible notes due 2026 [Member]	Mar. 31, 2009 Senior unsecured convertible notes due 2027 [Member]	Jun. 30, 2011 Senior unsecured convertible notes due 2027 [Member]	Jun. 30, 2010 Senior unsecured convertible notes due 2027 [Member]	Jul. 31, 2007 Senior unsecured convertible notes due 2027 [Member]	Jun. 30, 2011 Senior unsecured convertible notes due 2029 [Member]	Dec. 31, 2010 Senior unsecured convertible notes due 2029 [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]	Jun. 30, 2011 First mortgage loans [Member]	Jun. 30, 2010 First mortgage loans [Member]
Principal payments due on debt obligations
2011		$ 14,545,000					$ 14,545,000
2012		178,649,000				76,853,000	101,796,000
2013		574,860,000				300,000,000	274,860,000
2014		186,539,000					186,539,000
2015		431,087,000				250,000,000	181,087,000
Thereafter		4,948,379,000				3,838,077,000	1,110,302,000
Totals		6,334,059,000				4,464,930,000	1,869,129,000
Senior Unsecured Notes and Secured Debt (Textuals) [Abstract]
Senior unsecured notes		4,429,992,000		3,034,949,000	[1]	4,429,992,000
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	1,869,129,000
Senior unsecured convertible notes issued											345,000,000				400,000,000		494,403,000	450,000,000	450,000,000	400,000,000	600,000,000	400,000,000
Senior unsecured convertible notes, interest rate											4.75%				4.75%		3.00%	6.125%	4.70%	3.625%	5.25%	6.50%
Proceeds from issuance of senior unsecured notes		1,381,086,000	932,412,000															446,328,000	445,768,000			1,381,086,000
Conversion price of senior unsecured convertible notes											47.89				50.00		51.27
Number of shares per principal notes									20.8833				20			19.5064
Percentage of the principal notes to be purchased		1
Amount of principal notes		1,000
Senior unsecured convertible notes extinguished								5,000,000		214,412,000		5,000,000		226,914,000
Loss (gain) on extinguishment of senior unsecured convertible notes	(7,035,000)		(25,072,000)					446,000		8,837,000		594,000		16,235,000
Equity component of convertible debt, reacquire			(9,689,000)							18,552,000				21,062,000
Notes outstanding									125,588,000				168,086,000
Reacquired equity component of senior unsecured convertible notes																29,925,000
Assumed first mortgage loans secured	106,140,000		106,140,000																					689,876,000
First mortgage loans assumed		58,470,000
First mortgage loans interest		5.78%	7.35%																				5.388%
Number of properties secured by mortgages		32	17																				35
Additional Senior Unsecured Notes and Secured Debt (Textuals) [Abstract]
Secured debt		1,889,873,000		1,125,906,000	[1]
Carrying values of properties securing the debt		$ 3,215,570,000

[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.