Indebtedness Secured Debt (Details) (USD $)	12 Months Ended											12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jul. 31, 2012	Oct. 02, 2012 Fixed Rate Unsecured Debt [Member]	Sep. 30, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 15, 2012 Fixed Rate Unsecured Debt [Member]	Jun. 30, 2012 Fixed Rate Unsecured Debt [Member]	Aug. 31, 2011 Fixed Rate Unsecured Debt [Member]	Mar. 31, 2011 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Three Point Seven Five Percent Exchangeable Senior Notes [Member]	Dec. 31, 2012 Fixed Rate Secured Debt [Member] loans	Dec. 31, 2011 Fixed Rate Secured Debt [Member] loans	Dec. 31, 2011 Fixed Rate Secured Debt [Member] Minimum [Member]	Dec. 31, 2011 Fixed Rate Secured Debt [Member] Maximum [Member]	Dec. 31, 2012 Variable Rate Secured Debt [Member]	Dec. 31, 2011 Variable Rate Secured Debt [Member]
gross basis of secured properties	$ 2,000,000,000
Letters of Credit Outstanding, Amount	5,300,000
Secured Debt, Fixed Rate	4,891,881,000	4,117,230,000										1,251,477,000	1,256,331,000			18,386,000	6,045,000
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum												3.20%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum												4.70%
Number Of Secured Loans Assumed In Property Acquisition												9	13
Debt Instrument, Face Amount												96,100,000	162,400,000
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt												100,800,000	165,100,000
Weighted Average Interest Rate Secured Loans Assumed In Property Acquisition												5.56%	5.75%
Assumed weighted average remaining term												2 years 5 months 12 days	5 years 6 months 12 days
Debt Instrument, Interest Rate, Effective Percentage					5.45%	3.925%	6.01%	4.466%	5.69%	6.96%	5.62%	3.50%		3.50%	5.81%
Secured Debt, Variable Rate																13,300,000
Debt Instrument, Basis Spread on Variable Rate	1.25%			0.85%												2.50%
Debt Instrument, Interest Rate at Period End																2.71%
number of secured loans repaid												5
Repayments of Secured Debt	$ 117,287,000	$ 29,025,000	$ 207,060,000									$ 102,100,000
Repaid Debt, Weighted Average Interest Rate												6.08%