Summary of Significant Accounting Policies (Accounts Receivable and Accrued Rent Receivable/Deferred Costs/ Treasury Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Years	Dec. 31, 2010
Accounting Policies [Abstract]
Tenant receivables and accrued rent receivables, gross	$ 3.4	$ 3.7
Allowance for doubtful accounts receivable	$ 12.1	$ 11.6
Deferred leasing costs, amortization period, minimum	1
Deferred leasing costs, amortization period, ,maximum	15
Treasury stock, shares	0	116,679