Indebtedness Fixed Rate Secured Debt (Details) (USD $)	12 Months Ended									12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010 loans	Dec. 31, 2009	Aug. 31, 2011 Fixed Rate Unsecured Debt [Member]	Mar. 31, 2011 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2010 Fixed Rate Unsecured Debt [Member]	Apr. 01, 2010 Fixed Rate Unsecured Debt [Member]	Jan. 31, 2010 Fixed Rate Unsecured Debt [Member]	Dec. 31, 2011 Three Point Seven Five Percent Exchangeable Senior Notes [Member]	Dec. 31, 2011 Fixed Rate Secured Debt [Member] loans years	Dec. 31, 2010 Fixed Rate Secured Debt [Member] years	Dec. 31, 2011 Fixed Rate Secured Debt [Member] Minimum [Member]	Dec. 31, 2010 Fixed Rate Secured Debt [Member] Minimum [Member]	Dec. 31, 2011 Fixed Rate Secured Debt [Member] Maximum [Member]	Dec. 31, 2010 Fixed Rate Secured Debt [Member] Maximum [Member]	Dec. 31, 2010 Dugan Realty and Premier Portfolio [Member] Loan One [Member]
Weighted Average Interest Rate Secured Loans Assumed In Property Acquisition										5.75%
Assumed weighted average remaining term										5.5	1.9
Number Of Secured Loans Assumed In Property Acquisition		19								13
gross basis of secured properties	$ 2,000,000,000
Letters of Credit Outstanding, Amount	6,200,000
Secured Debt, Fixed Rate	1,300,000,000
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	3.10%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	6.10%
Debt Instrument, Face Amount		479,000,000				279,900,000	250,000,000			162,400,000
Liabilities Assumed	177,082,000	527,464,000	0							165,100,000
Debt Instrument, Interest Rate, Effective Percentage				5.69%	6.96%	6.26%		5.37%	5.62%			3.50%	5.00%	5.81%	5.50%
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt	$ 283,241,000	$ 484,700,000				$ 22,400,000
Debt Instrument, Market Interest Rate at Acquisition																6.46%